When Sebastian Coe unveiled the design of the 2012 London Olympics stadium in November, you could have heard a pin drop. There was a breathless hush - and stifled yawns. Was this really it? The architect was smooth-talking, but as I looked around, the mood of photographers, cameramen and journalists said it all. It might well do the job, but its design seemed almost wilfully lacklustre.  In dramatic contrast, the main stadium for this year's Beijing Olympics is, quite simply, stunning. Here is an adventure in steel and concrete, a building - despite its age-old purpose - like no other. Its structure is very nearly complete, while the fit-out, with its plethora of shops, restaurants, cafes, bars and meeting places, is racing ahead. At times, there have been as many as 7,000 construction workers on site, yet this is no rush job. It is a work of exceptional quality.  If China has set out to impress the world with the 2008 Olympics, the stadium and its attendant buildings - the Aquatics Centre and Digital Beijing (the Olympics "command post") - have set a heady precedent. The furtive London Olympics, with its glum building site hidden behind what can only ever be known as the Great Wall of Stratford, will struggle to match the design standards set by the Chinese, although Zaha Hadid and her wave-like swimming pool offer a ray of hope for the London event.  At the moment, although the Beijing stadium can be seen from the long avenues flanking the Olympic Park, access is barred to all but the most determined. Guards in thick green army coats and fur hats patrol the gate, while workers - from the far-flung provinces, mostly, and earning about £3 a day - troop in and out. Inside the compound where the stadium sits, its structure now complete, men stand on high-level walkways brandishing welding guns. Others balance precariously from the steelwork; at least 10 are said to have fallen and died here.  An eye-catching design even from a distance, the stadium proves to be a mind-bogglingly complex artefact as you get up close, and it is just as dizzying on the inside. The Chinese named it the Bird's Nest as soon as its creators - Swiss architects Herzog & de Meuron, working with Arup and the China Architectural Design and Research Group - unveiled their design five years ago. It measures 320 metres by 297 metres, and is 69 metres high. Its mesmeric steel frame, 41,875 tonnes in all, loops, swoops and swirls over and around the great, red, concrete 91,000-seat arena. What had seemed to be a solid structure from a distance proves to be a filigree Chinese puzzle close up.  Jacques Herzog has described this 12-metre-deep frame as "an architectural forest", its beams, stairs, frames and other steel elements standing in for trees and branches, and providing an awe-inspiring transitional space between the exterior, the Olympic Park, and the interior, where the arena awaits. The latter, a great, red bowl of concrete, proves to be almost circular, so everyone is equally close to the running-jumping-vaulting action. This makes it just that bit different from other ambitious stadiums, such as Wembley, and it does feel that bit more of a piece, streamlined even. Above the tiers of plastic seats and confident-looking clocks, the roof is a simple membrane designed to keep out sun and storm (the Olympics take place in August, when Beijing is hot and thundery) and to act as a foil to that intricate steel skeleton.  When the Games are over, the stadium will host sporting and cultural events, as well as rock concerts and those ineffable flag-waving parades so beloved of the Chinese. By then, the stadium will sit comfortably in landscaped gardens, also designed by Herzog & de Meuron, intended to be contemporary plays on traditional Chinese themes. If all goes to plan, the Olympic Park might yet turn out to be an Unforbidden City, a people's palace gardens of the future.  Not everyone likes the Bird's Nest, though - least of all Beijing artist Ai Weiwei, whom Herzog & de Meuron persuaded to work on the design. Ai Weiwei, who once smashed a 2,000-year-old Han dynasty vase to make a decidedly iconoclastic point about Chinese art, and who, in 2000, curated a highly controversial art show in Shanghai entitled Fuck Off, was raised in a labour camp in Xinjiang province. In the 1950s, his father Ai Qing, one of China's most celebrated modern poets, was denounced as "an enemy of the state and a rightist". Ai Weiwei tends to speak his mind, even in a country that still frowns on free expression. He likens the 2008 Olympics to "a pretend smile" and, although he believes the stadium to be beautiful, he also calls it a "public relations sham" hiding the true political nature of China. His stance serves as a reminder that this global sporting event has not always gone hand-in-hand with polite politics and international camaraderie, however great its architecture and athletes.  The Bird's Nest sits near the second of the major Beijing Olympic venues, the Water Cube; or, to give it its official name, the National Aquatics Centre. This magical structure - designed by Australian architects PTW with Arup, the Shenzen Design Institute and the China State Construction and Engineering Corporation - has just opened and is already hosting a Chinese swimming competition.  In essence, the Water Cube is a big, £50m steel and concrete box containing a huge swimming pool flanked by 17,000 seats. But what an extraordinary box it is. The pattern of the structure has been designed to hold huge pillows of ETFE, a lightweight substitute for glass, the same inflated hi-tech material used to clad the domes of Cornwall's Eden Centre. The pattern is based on the natural formation of soap bubbles to give a random, organic appearance.  It was clearly clever, not to mention fun, to make a big box into an architectural and engineering conjuring trick. Yet this is also a responsible design; the pillows heat up in the sun, and this in turn heats the water in the pool and feeds into the rest of the building. It is exceedingly efficient. In the evening, as an added bonus, the Water Cube glows a happy, swimming pool blue; it looks great from the street, one of those buildings you feel compelled to go and have a look at.  The last of the three major Olympic buildings was designed by Zhu Pei, a Beijing architect trained in the US as well as China. This is Digital Beijing, the control and data centre for the Games, scheduled to become a museum and exhibition centre for digital design and technologies and, apparently, a communications hub for the city. Zhu's design, in stark contrast to the stadium, is a sequence of tall buildings that look, on first encounter, rather like upright 1960s IBM computers.  What makes it a delight, as you get up close, is Zhu's creative use of unexpected materials. A special fibre he developed with a Chinese manufacturer will be used for floors in the building, and is ideally suited for the projection of ever-changing digital images. Equally, the material is strong enough to be used for sci-fi-style pedestrian bridges within the building. As for the oddly gleaming end walls of the pavilion, these are composed of aluminium sheets made to look, strangely, like stone. The technique was developed by a Chinese tin can manufacturer. Ingenious - and the result is delightful.  Zhu is one of a number of Chinese architects who have been able to set up their own studios freely in recent years, and who are keen to invent and develop a modern Chinese architecture that plays with, rather than mimics, tradition, one that is as independent as it wishes to be from contemporary western practice. The great luxury Zhu has enjoyed with the Olympic project is time. Most modern Chinese architecture is raced up as fast and as cheaply as possible, leaving precious little time for original thought or craftsmanship. Yet the three major Olympic buildings have been almost five years in the making.  And they are not just for the summer Games. Each has something to offer a new, specifically Chinese architecture that might yet emerge - against the political and economic odds - in the coming years. For this alone, each deserves a gold medal, though the spellbinding Bird's Nest deserves a special award of its own. The designs will be on show at the V&A's China Design Now exhibition, opening in London next month, but nothing quite prepares you for the reality. Herzog & de Meuron, along with an army of Chinese workers, have conjured a truly Olympian design.  · China Design Now is at the V&A, London
In a special feature on the architecture of the Beijing Olympics, Jonathan Glancey sneaks a look inside its signature building - the looping, swooping, stunning stadium