In 1969, a year after Russian tanks rolled into Prague, Josef Koudelka visited London with a Czech theatre group. One Sunday morning he was walking out of his hotel near the Aldwych Theatre when he saw some members of the theatre group perusing a copy of the Sunday Times magazine. As he passed, he saw to his surprise that they were looking at his own extraordinary photographs of that Russian invasion and the spontaneous street protests it provoked. The same photos have since become the definitive pictorial record of a pivotal event in 20th century history.  'They showed me the magazine where it said that these pictures had been taken by an unknown photographer from Prague and smuggled out of the country,' he says, shaking his head as if he still cannot believe it. 'I could not tell anyone that they were my photographs. It was a very strange feeling. From that moment, I was afraid to go back to Czechoslovakia because I knew that if they wanted to find out who the unknown photographer was, they could do it.'  Before he returned to Prague, Koudelka began making preparations to leave again, this time for good. He began by contacting the Magnum photographic agency, which had placed his work in the Sunday Times, attributing the images to PP (Prague Photographer) lest he and his family should suffer reprisals. Magnum subsequently wrote a letter to the Czech Ministry of Culture saying they had given him a grant to photograph Gypsies across Europe. It worked. In 1970, when his visa ran out, Koudelka did not return home. Suddenly stateless, he too became a kind of Gypsy, constantly on the move, forsaking the very notion of a homeland.  So began perhaps the most remarkable journey in 20th century photography. When I meet him today, in the back room of his new apartment in Prague, Koudelka unfolds a battered map of the world he has just found in one of the many boxes stacked along a wall. It is covered in spidery ink trails that trace his wanderings through Europe and beyond, his handwriting providing a runic commentary of the festivals and gatherings he attended along the way. The map dates from the Seventies and looks like a strange work of art, which, in a way, it is. The real art, though, lies in the photographs Koudelka produced when he began chronicling his restlessness - and rootlessness - as well as his newfound sense of freedom. His first major work, published in 1975, was called simply Gypsies, his second, from 1988, Exiles. Their titles alone tell you much about Koudelka's own life as well as the lives of his subjects.  'For 17 years I never paid any rent,' he says, laughing and raising a shot glass of slivovic, a plum brandy he has produced to welcome me to Prague. 'Even the Gypsies were sorry for me because they thought I was poorer than them. At night they were in their caravans and I was the guy who was sleeping outside beneath the sky.'  Now, 38 years after he began his exile, and 40 years after the invasion, Josef Koudelka has a place in Prague that, were he so inclined, he could call home. It is an apartment on the corner of a quiet square near the city centre, opposite a church where, he tells me proudly, the young Dvorak once wrote music. It is light, airy and purely functional: a place to work rather than rest. The boxes of notebooks, meticulously classified, suggest a highly ordered mind. 'Josef is utterly methodical, and utterly aware of his own legend and his legacy,' says his close friend, the photographer, Eliott Erwitt. 'But he is also an eccentric. He thinks differently, and, as we know from the work, he sees the world differently.'  At 70, Koudelka has, like his late friend Henri Cartier-Bresson, achieved semi-mythic status as a photographer. Alongside Robert Frank, he is the last of the great hard-bitten romantics of 20th century reportage, and, like Frank, he is a hero of mine. Gypsies was the first photography book I ever owned, and though I cannot remember now how I came by it, I can still recall its impact on me. I was studying in London for a degree in English, and Gypsies seemed to me to possess a more powerful narrative than many of the contemporary novels I was reading. I looked at it again in preparation for this interview, and found it still retains the power to mesmerise with its raw beauty, its essential sadness. There is something beautifully melancholic in Koudelka's images, a sadness the Portuguese call saudade, a deep-rooted longing for which there is no equivalent word in English.  When I mention this, he nods in agreement. 'The mother of my son, an Italian lady, she once told me, "Josef, you go though life and get all this positive energy, and all the sadness, you just throw it behind you and it drops into the bag you carry on your back. Then, when you photograph, it all comes out." Perhaps there could be some truth in that.'  Koudelka, his youthful face framed by white straggly hair, has the demeanour of someone 20 years younger, and proves to be a generous and engaging host. He offers me his old flat in the suburbs of Prague to stay in, and insists on paying for lunch and drinks two days running. He famously does not grant many interviews and, one senses, astutely maintains his own mythology by his silence. Today, though, the anecdotes flow as freely as the wine.  Koudelka has returned to Prague from his adopted home in Paris for an exhibition of his '68 photographs that is being held in the Old Town Hall to coincide with the 40th anniversary of the invasion. A book called Invasion 68: Prague was published earlier this year, an extraordinary chronicle of those convulsive seven days when the people of Prague, young and old, took to the streets in their thousands to protest at the Russian presence.  'For a long time no one here was interested in remembering,' says Koudelka, 'but now I think they start to remember again. If this book helps the remembering I am happy. We Czechs are not like you Irish or the Poles. We do not behave bravely many times against the odds, but in this one week, as my book shows, we should be very proud of how we behave.'  In that one week, the 30-year-old Koudelka took over 5,000 photographs on the streets of Prague, often under extreme conditions. He was shot at by a Russian soldier, and pursued through the crowds and into the backstreets around Wenceslas Square. In the short film that ends the exhibition, a young and scrawny Koudelka is seen perched on a Russian tank, filming a young Czech man waving a flag of resistance.  Some of the people in the photographs turned up for the opening of the show, including a man whose arm Koudelka had positioned in the foreground of an empty Prague street, the watch on his wrist telling the time of the invasion. 'Forty years have gone by,' says Koudelka, 'and I do not remember them nor they me. You cannot rely on your memories - but you can rely on your pictures.'  Back then, he says, he did not even know the word 'photojournalism'. 'I had not seen Life or Paris Match. For me, it was just important to photograph what I was part of.' To this end, he used a primitive Exakta camera and countless rolls of cinema film that he had bought cheaply from a friend, and which he cut into strips, then draped over his shoulder and fed into the camera. 'I used to have to run home to reload, always thinking I was missing something. Once, I think it saved my life. I was reloading when I heard the explosion that killed many people outside the radio station.' Until now, remarkably, the world has only seen 10 of those images.  'Originally, I did not want to make the book or the exhibition,' he says. 'I knew already I had selected the 10 best. And, to be truthful, when I was working on this book, I did not discover one that I would have added to these 10. They are the ones that have a universal value. In them it is not so important who is Russian and who is Czech. It is more important that one man has a gun and one man has not.'  The previously unseen images, though, are powerful for different reasons. They provide a unique visual record of 20th century history as it unfolded, an event of incredible emotional and political intensity that lasted a week yet changed the course of a nation for two decades afterwards. In one brilliantly composed image after another, Koudelka has also captured the energy of mass protest like no one before or since, an energy that flared from angry disbelief into violent protest and then faded just as suddenly. 'When I look at these pictures now, I can't believe I did them. Magnum could not believe it was the work of one photographer. I was everywhere because everywhere I looked in those seven days there is another photograph waiting for me. I did not have time to even think about the danger.'  In one unforgettable image he snapped a tiny old man on his way to work, briefcase in hand, who suddenly stopped to hurl a cobblestone at a Russian tank. In another, he captured a young man holding his coat open as if inviting a nervous soldier on a tank to shoot him in the heart. In another, a pretty woman pleads with the Russian commander who stares over her head, implacable, imperious.  'In truth, I felt no hatred for these soldiers,' he says now. They were not responsible for this. They were young like me, and we were in the same system. What happened to them could easily have happened to me. They could have woken me one morning and sent me to Warsaw or wherever they wanted. Back then, none of us was free.'  The pursuit of freedom is what has fuelled Koudelka's life ever since, and that has manifested itself in a desire to be constantly on the move. Initially, when he began living in London in 1970, unable to speak English very well, he used the large Bayswater flat of fellow Magnum photographer David Hurn as his base.  'It was nicknamed the Dosshouse,' says Hurn, 'because every struggling photographer passed though there at the time and slept on the floor. Josef was brought there by Elliot [Erwitt], who had been involved in helping him get out of Czechoslovakia, and the understanding was that he would be staying a few weeks. Like everyone else, he asked if he could use my darkroom to develop some film. He turned up with 800 rolls. I think, in the end, he stayed there off and on for about nine years.'  London was his base in the winter but, come the summer, he would head off on his travels across Europe, rucksack and sleeping bag slung over his shoulders. He photographed Gypsies at the Epsom races and Appleby Horse Fair, pilgrims on St Patrick's Mountain and Lough Derg in Ireland, and itinerant musicians in Spain and Portugal, always travelling light and with little money, always sleeping in the open air. Like all great photographers he possesses an eye for telling detail and a formal mastery that, even in his snatched reportage, seems uncanny in its compositional brilliance. In one startling image that has haunted me since I first saw it, a bewildered young Gypsy in handcuffs stands alone on a hill while in the middle distance a line of people, and a few uncertain-looking policemen, stand watching like onlookers in an absurdist drama. The man, it turns out, had just been arrested and charged with murdering his wife.  The Gypsies he photographed called Koudelka the 'romantico clandestino'. 'I photograph only something that has to do with me,' he says, 'and I never did anything that I did not want to do. I do not do editorial and I never do advertising. No, my freedom is something I do not give away easily. And I do not follow the war because I am not interested in photographing violence. Sure, if I was in Georgia now, I would be photographing what happens.'  He stops, shakes his head, and reaches for another glass of wine. 'You know, people say, "Oh, Josef, he is the eternal outsider," but on the contrary I try always to be an insider, both as a photographer and as a man. I am part of everything that is around me.'  Koudelka was born in a village of 400 people in Moravia in 1938, close to where Milan Kundera, that other famous Czech exile, grew up. As a child, he was fascinated by folk music, planes and the family photographs the local baker took on a Bakelite camera. 'Once I saw the bread man's camera, I went off every day to the woods to pick wild strawberries to sell to the man who made ice-cream. In this way I saved the money to buy a camera of my own.'  When he moved to Prague to study engineering, Koudelka fell under the influence of Jiri Janicek, who ran a photographic club for students. Soon he was contributing to a theatre magazine called Divadio, where he published his first serious body of work: intense, close-up portraits of the actors in rehearsal. Until 1967, though, he worked as an aeronautical engineer in Prague and Bratislava. 'It was fantastic because I loved aeroplanes as much as photographs,' he says, 'but there came a point where I knew that if I stayed in engineering, I would surely die. I had no ambition to be the boss, and I had all the responsibility for the people's safety. With a camera you are your own boss, responsible only to yourself. You are alone with the camera, there is no one else to blame.'  Until the upheaval of Prague '68, Koudelka sought out and photographed Czechoslovakian Gypsies. Afterwards, in exile, and now a full-time member of Magnum, he forged an unlikely friendship with the agency's founder, the great French photographer, Henri Cartier-Bresson, and lived on small grants and awards while he searched out Gypsy communities across Europe. Between travels he would turn up out of the blue at the Magnum office in Paris and sleep there for weeks under a desk. His late father, he tells, me, used to send him home-brewed slivovic every month. 'It was a plastic bottle with the word vinegar written on it in Czech. This way, he thought I would not have to pay duty on it.' He became known around the Magnum office as 'Le Remplisseur Professionnel' (the professional pourer). 'After the first glass, no one can breathe,' he says, cracking up at the memory. 'After two, the girls are dancing around the floor.'  Koudelka speaks without regret about his constant travelling. 'In Britain the Gypsy children would always ask me two questions: "Do you sleep dressed?" and "When was the last time you saw your people?" They were nomads but they returned to the same places every year and caught up with their own people. I think they pitied me because I could not do that.'  When I ask David Hurn if Koudelka ever seemed homesick, he replies: 'Josef is not the sort of man to dwell on stuff like that but he would sometimes fall silent and take himself off somewhere.' He would often disappear into the kitchen to make mashed potatoes with caraway seeds. 'He seemed to live on this one dish, and, like his insistence on sleeping on the floor, I initially thought it was a kind of romantic affectation. Then one day his mother turned up from Prague to visit. She could not speak a word of English. One afternoon she said something. I asked Josef to translate, and he said, "My mother just said how much she would love a big plate of mashed potatoes with caraway seed."'  Though he has never maintained a long-term relationship, Koudelka has three children by three women in three countries: two grown-up daughters and a son of 13. Recently, when a teacher asked the boy what his father did, he replied, 'He's a nomad.'  When pressed, Koudelka talks with pride about his children, but one detects regret too. 'Listen,' he says, when we meet the following day over a beer, and I broach the subject of family and commitment, 'I am not a family man and I can never be a family man. But I am very happy, I have children and I hope that they are happy that they exist. From the beginning I make certain rules with my children and one is that I can't be with them all the time. I tell them that when I am with them, I am for them, and when I am not there, it is best they should try to forget that I exist.'  This flintiness exists side by side with his generosity of spirit, but essentially, one suspects, Koudelka is indeed a nomad and one of life's loners. Though he does not let people get too close he maintains many deep and abiding friendships. But one senses that his life, like his photography, is undergoing a profound change, and the new apartment in Prague is only the most visible sign that he can no longer maintain the nomadic lifestyle that framed his restless art and the arc of his younger life.  In the last 10 years or so people have disappeared from his photographs altogether, and he now works obsessively on panoramic landscapes that often portray the devastation industrialisation has wreaked on the natural environment. He has two shows coming up in New York at the Aperture and Pace MacGill galleries, and he is also completing a long-term project that records the changing countryside near Piedmont, and considering a commission that would take him to Israel to document the so-called security wall that has annexed more land from the Palestinians.  'If I go,' he says defiantly, 'it will be to photograph the wall not as a political problem but because of its devastation of the landscape. And this is monumental. I see this wall as a failure of civilisation. That is a subject for sure, but I need to know how the images will be used, and if I have creative control. This is one of the many important things I learn from Cartier-Bresson - you must always ask where the money comes from.'  As we wind down over a last beer in a bar near his flat, I sense that Koudelka has grown impatient with all this questioning, but the mention of Cartier-Bresson sets him off on another bout of reminiscing. Earlier he had shown me a letter of introduction the French master sent to American photographer Cornell Capa ahead of Koudelka's first trip to the US. It says simply: 'I am sending you a madman called Josef.'  What was the basis of his friendship with Cartier-Bresson? Koudelka thinks for a long while: 'We fight and disagree all the time,' he says, throwing back his head and laughing. 'He tries to tell me how to think, and I tell him exactly what I think, then he shouts, "How can I have such a man in my house?" This is how it could goes with us, and then it would be quickly forgotten. As a photographer I do not think he influenced me, but he helped me to understand how the profession worked. In fact I have learned so much from him, as a man, that it is impossible to put in words. He questioned everything all the time, and he was so full of generosity to me.'  Not for the first time today, Koudelka's eyes fill up, but he drinks some more beer, and continues: 'Once, Henri rang me in Paris and said, "Josef, Kertész is in town, you must come to dinner and meet him." He held Kertész in the highest regard as a photographic master. I said, "Henri, I love his pictures but I do not need to meet him." The phone goes down. Then he rings back and says, "No, you do not understand, you have to meet him because we three, we are of the same family." At the time, this seems to me to be an unbelievable thing to say. Now, though, when I look back from a distance, I can see that maybe there is something in that.'  · Invasion 68: Prague is published by Thames & Hudson at £29.95. To order a copy, go to observer.co.uk/bookshop or call 0870 836 0885  1938 Born in Boskovice, Czechoslovakia.  1952 Aged 14, he is introduced to photography by the local baker. He saves up for a camera of his own by picking strawberries and selling them in the village.  1961 Graduates from the University of Technology in Prague and begins working as an aeronautical engineer. He also stages his first photographic exhibition.  1962 Begins photographing Gypsies, a project he would continue for several years.  1963 Starts to take commissions from theatre magazines and regularly photographs productions at Prague's Theatre Beyond The Gate using an old Rolleiflex camera.  1967 Gives up his career in engineering to work as a full-time photographer.  1968 During the Russian invasion of Prague on 21 August, he takes the most important photographs of his career while perched on the roof of a building in Wenceslas square. The pictures are later smuggled out and published under PP (Prague photographer).  1969 Wins the Robert Capa Gold Medal Award for his invasion photographs; the prize is dedicated to 'an unknown Czech photographer'.  1970 Flees to England where he applies for political asylum.  1971 Joins the Magnum picture agency, where he would stay for more than decade.  1975 Gypsies, his first book, is published alongside an extensive exhibition of his photographs at Moma in New York.  1978 Wins the Prix Nadar.  1984 The Hayward Gallery, London, presents an exhibition of the Prague invasion pictures, crediting Koudelka as the photographer for the first time.  1988 Exiles, his second book is published.  1986 Invited by the French government to document the urban and rural landscape of France.  1987 Becomes a French citizen.  1991 Returns to Czechoslovakia for the first time and begins to photograph its wasted landscape, which would become Black Triangle. Wins the Henri Cartier-Bresson International Grand Prize.  Over the past decade, Koudelka has continued to live and work in the Czech Republic and France.  · This article was amended on Sunday August 31 2008. Invasion 68: Prague, a photographic chronicle of the Russian invasion of Czechoslovakia, is published by Thames & Hudson in the UK, not Aperture, as we previously said. This has been corrected.
Forty years on from the 1968 Soviet invasion of Prague, we meet Josef Koudelka, the man who captured the most  startling images of that dramatic week, then went on to become one of the greatest photojournalists of our time