No portrait is more important to Xu Weixin than his first. It was 1966; the artist was eight; and he had learned, to his shock, that his kindly young teacher was the daughter of a landlord – an enemy of the people. Outraged, he drew a hideous caricature and pinned it to the blackboard. When Miss Liu entered the classroom, "She turned pale but didn't say a word," he said.  She had good reason to be frightened. The Cultural Revolution was at its height, and across China teachers, former landlords and intellectuals were being humiliated, beaten and murdered. They were hounded by neighbours, colleagues and pupils moved by misguided revolutionary fervour, personal grudges or little more than whim. Friends, children and spouses turned on them.  By the time the chaos subsided 10 years later, an estimated 36 million had been persecuted and at least 750,000 were dead in the countryside alone. Red Guards had smashed up temples, burned books and destroyed historical treasures. Universities had closed and pupils missed years of schooling. Even Communist party historians describe it as a disaster, unleashed by Mao Zedong. But their terse verdict is designed to pre-empt, rather than encourage, debate. An event that defines China to this day – that helps to explain its fixation with political stability; its dramatic economic reforms; even, some say, its increased individualism – remains largely taboo.  Xu, now 53, is among the handful daring their country to confront its past. The faces of the Cultural Revolution are captured in the immense black-and-white portraits stacked in his Beijing studio. Each standing 2.5m tall, they are both personal and powerful, demanding attention. The monochrome oils are in stark contrast to the garish colours of 60s propaganda.  Some of Xu's subjects were victims, some perpetrators. Many were both. Mao is there, as is his infamous wife Jiang Qing; so are unknown scholars and Red Guards. It has taken the artist five years to complete this series of just over 100 paintings. But it is work he has been preparing for all his life. "I feel they are related to that first portrait," Xu says. "I feel guilty [about my teacher]; but it also helps me to understand… People who were close to you – who were friendly and kind – could suddenly turn upon you."  The theory was that creative destruction would eradicate old habits and ideas, transforming a struggling country. More urgently, the disastrous Great Leap Forward and Khrushchev's fall in the Soviet Union impelled Mao to see off rivals and critics. His heir apparent Liu Shaoqi was one of many to die in disgrace.  The violence shook every strata of society and rippled out to the farthest corners of the country. Teenagers and youths were encouraged to attack fellow citizens. More than one observer has compared the anarchy to Lord Of The Flies.  "It's very, very vivid," Xu says. "I remember all the demonstrations and public denunciations; people breaking pictures and smashing Buddhas. At the beginning, people were using bricks or wooden rods and metal bars to hurt people. We could hear gunshots at night and people were beaten to death." His hometown in Xinjiiang was far from the worst affected. In Chongqing, rival factions battled with guns and tanks. In Guangxi, there are accounts of cannibalism. Victims were condemned as "monsters and freaks"; Xu's response is not to demonise their accusers, but to approach each subject with the same neutral gaze.  "Even if they are bad people, they are still people. I have to respect them," he says.  As a child he, too, believed the Cultural Revolution was "a great thing, a right thing, and something we must do". In retrospect, the movement was not just horrific but often ludicrous in its paranoia: the most "sinister" aspect of one supposed conspiracy, notes the book Mao's Last Revolution, was that even some of its core members appeared unaware of its existence.  China's current leaders undoubtedly understand the damage; several of their parents suffered, even died. But a fuller reckoning of events – and Mao's role – would risk undermining the party's hold on power.  "In textbooks this long period of history is described with one sentence, and you can't discuss it," says Xu, who believes it has become harder to talk about over the last decade.  Several of his portraits were exhibited in Beijing a few years ago, but he does not expect another show on the mainland. He merely hopes more people will become aware of his work, and reflect on their own experiences. "Most people think the Cultural Revolution was the Gang of Four's fault, but actually everyone should be responsible." That includes the eight-year-old who scrawled his teacher's picture. That Miss Liu survived the decade largely unscathed is some comfort, Xu says, but, "Of course, I was responsible. It's only a question of how great or small my responsibility was."  Almost half a century on, it floods back: the hope, the zeal, the carefree autumn days riding the rails with fellow teenagers. And with it comes the shame, the fear and the blood clotted on a dying man gasping for water.  Yu Xiangzhen was an idealistic 14-year-old when the Cultural Revolution broke out, and among the first to form a Red Guards group. "We were taught that Chairman Mao was closer to us than our parents – he was like a god to me," she says. From the first, she had doubts: when she saw fellow students berating and humiliating teachers, hacking off their hair and pouring glue over them; when she watched her peers assaulting "capitalists" and "rightists". It felt wrong, and yet, "I still thought it was right because everything I was hearing was that we needed to break the old world to build a new one.  "I didn't think these people deserved to be beaten up....[But in refusing to take part] I felt I was, indeed, not brave enough. It was a loss of face."  Then, she says, came "something so horrifying I will never be able to forget it as long as I live". There is no doubt she is still traumatised, and her voice rises to a shriek as she describes it. "It was dark – I was standing by the side of a road, waiting for my friends. I heard someone whispering for water and saw a man crawling towards me from the basketball court," she says. "He was covered in blood. The blood on his head had congealed already. I was terrified. Then I saw the court – it was almost covered by dead bodies." All, she believes, beaten to death by Red Guards.  Yet, for these teenagers, it was a heady as well as a frightening time. Hours after witnessing the atrocity, Yu was on a train to Shanghai. They were travelling first to spread the cause – bearing leaflets titled "Long live the red terror" – but then "it just became travel and leisure". Trains were free to Red Guards; food and lodgings awaited them. "There were no plans, no destinations… I was just very happy."  Yu has begun to blog about her past in an attempt to understand it. "I turn 60 very soon. There isn't much time left to think properly and write," she says. But she struggles to make sense of the violence, and few friends want to discuss it. "The Red Guards who were most active had [political] problems in their family and tried to prove they were different," she suggests. "Every time we get together, I look for the people who were most brutal. One told me it was exciting to go to people's houses and smash things and beat them up. You felt you could do whatever you wanted – that you were in control… And you thought it was the right thing to do."  Chow is a modern Chinese success story. She returned six years ago, from the States, lured back by the country's transformation. Her company sells cupcakes and confections to Beijing's elite. And yet there is a space in her life so profound that, "I don't know what it would even mean to have that gap be filled; I would be another person."  It was left by the handsome young man hanging on the wall of Xu's studio: the father she knew for a few months before he was taken away. Zhou Ximeng killed himself in captivity, aged 27. "It's painted like a memory. It's like he's frozen in that time," she says.  She knows her father from a handful of photographs and from the stories her mother has told, of a smart, confident, capable man – too accomplished, perhaps. "My mother said he was an overachiever.," Chow says. "Whatever he did, he excelled at – he was always top of his class. The reason she gave for his suicide was that he had never encountered any huge obstacles. I think he reached a point where it was all beyond his control and he didn't feel he could change anything. You had to first renounce yourself and then renounce your family and friends. I think, when he got to that point, really, he just closed up."  Zhou came from a long line of landowners and scholars; his father, a renowned paleontologist, had spent time in America and Taiwan. But in the Cultural Revolution such a privileged pedigree condemned him. All it took was "some really small comment" for him to be seized and held, in a village outside Beijing. His body lies somewhere near the train tracks where he died.  Xu has painted Zhou's mother, too; another of the casualties: she never recovered from her son's death and killed herself years later.  Chow and her mother moved to Hong Kong – and later the US – as soon as China began to open up after Mao's death. Friends were equally keen to leave but too frightened to apply thanks to the previous decade's frequent reversals: "They say one thing, you do it, then they say, 'You guys are going to jail because you have revealed your true selves.'"  Her father was subsequently forgiven "for his crimes, whatever they were", she says. "I don't feel bitter or angry – I feel sad for him, that he missed so much," adds Chow, now 42, and who has two daughters.  She wants the next generation to comprehend what happened. "I don't think they really know about it or understand it or even talk about it. It's important for them to know what their ancestors went through and what was lost."  When Liang Sicheng was denounced as a counter-revolutionary, he was scared to look even his wife in the eye. Lin Zhu, who had been working in the countryside at the time, rushed home to him on learning the news.  "He said, 'I've been waiting for you and missing you every day, but I'm afraid to see you,' " the 83-year-old recalls, reaching for a tissue to wipe away tears. Her husband sensed the horror ahead. Beijing's Tsinghua University – one of the country's top institutions – was already covered in posters attacking professors.  "Back then, I thought this was like a dark cloud that would soon pass. I didn't realise it would cover the country for the next 10 years," Lin says. When it lifted, Liang was dead, his health wrecked by the scores of lengthy "struggle sessions" publicly to humiliate him; by beatings from Red Guards; and by the cold, damp conditions of the building to which the family had been moved.  Lin still struggles to understand how hundreds of millions could participate in such cruelties. Some of Liang's persecutors were forced into taking part, she says; others were jealous of his success. Most were young students who did not understand his ideas. To her husband, who had loved teaching, that was particularly painful.  "He wrote confession letters, one after another, but didn't know what he had done. The most important claim was that he had received a 'capitalist education'. No one could tell us what proletarian architectural design was – and you were too afraid to ask."  As the movement escalated, Lin considered demands to join it: "I thought probably I would be beaten to death by the Red Guards. Maybe my children would desert me and my friends would keep their distance. But I couldn't understand what Liang Sicheng had done. I couldn't go against my conscience by leaving him."  Together they endured six years of enforced Maoist study and public denunciations that often ran for hours. "Because it was all day long, the brain sort of became numb," Lin recalls. "Normally he was not beaten up at those sessions, but sometimes they would come and beat us at home."  Liang's ordeal ended when he grew so sick that he could no longer rise from his bed for the struggle sessions. He died in 1972, aged 70.  In later years, Lin worked with her husband's accusers; some, quietly, apologised. She does not blame individuals for caving into pressure to attack others, though she is adamant that she never did so. She even suggests those years helped her to grow. "Whatever happens, whatever comes, I'm not afraid any more. It made me stronger and made me think," she says.  But she fears that intellectual life in China has never fully recovered – and she worries the country could see another such movement. "Many of us are concerned about whether we can avoid a similar disaster in future. History doesn't repeat itself exactly… but it's possible."
It was a time when pupil turned on teacher, when friend turned on friend… Now artist Xu Weixin has painted both victim and perpetrator. Tania Branigan reports