A city street at first light, a camera, 25 volunteers in the nude
It is 6.30am on a Saturday morning and 40 people are clustered in the foyer of the Holiday Inn, Piccadilly. We are all waiting for the sun to rise. When it does, 25 people are going to run out on to neighbouring Berkeley Street, strip off and lie on the road stark naked. The other 15, an assortment of journalists, amateur cameramen and photographers, will follow, but remain fully dressed. The Holiday Inn staff have no idea why we are here. They are just serving us coffee.

The people soon to be naked are huddling together. They all look a bit nervous. Although we call them models, this is no professional shoot; they are simply volunteers who want to be part of photographer Spencer Tunick's latest project.

Spencer Tunick is a 28-year-old New Yorker who earns a living selling burritos. However, his alternative career has made him something of a cult figure; Spencer likes to photograph ordinary people in urban settings. Naked ordinary people, that is. "I suddenly thought, why can you take nude pictures in a studio, but not in a city with real backdrops, which are far more interesting?" says Spencer, who has so far evaded arrest for promoting indecency, and has inspired punters to reveal all in New Orleans, Los Angeles and New York.

In London for the opening of his new show, Spencer decided to herald his arrival with aBritish shoot; the phone line for would-be models has been hot for the last two weeks. There is no money, but anyone willing to strip off will receive a print of the resultant photograph.

In the cold not-so-light of morning, however, some of the strippers don't seem so wild on Spencer's big idea. "I've got a big bite on my leg. It's inflamed," says Jonathan Lewis, who works in the V&A and admits he's never gone naked in our capital before. "I hope Spencer won't mind. I'm sure he'll notice it."

"It's the cold. That's what we're all worried about," says a man called Colin, who reluctantly admits he works for the Financial Times. "All the men are worried, that is. Getting out of the shower is one thing, but here you won't have that, erm, that hot-water feeling when you hang low." Colin coughs, and looks anxiously around. "This is the kind of cold that makes your willy disappear," he whispers.

For all the male insecurity, this is very much a boy's event. Men outnumber women by about 10 to one. "Maybe we have a deep-rooted need to expose ourselves," says Colin. Maybe men are just more confident. "I've been careful about what I ate yesterday," says Wendy Ide, one of the few female volunteers. "I don't want any unsightly stomach bulges." Her friend Henry Johnson gives me a look of hilarity. "Have I been shaping up? For this? If you mean getting totally pissed last night, then maybe."

"Have you all remembered not to wear underwear?" shouts Fiona Mosely, gallery curator and shoot organiser. "Spencer doesn't want any red underwear marks. And could you remove glasses, and all jewellery?" She turns to Spencer. "Do you want nose studs out?" Spencer ruminates for a minute. "Yeah," he finally says. "This one's not about piercings. So everything out."

It is getting lighter by the minute. Spencer addresses the crowd. "Will you get into groups of five, and run outside, please," he announces. "Very quickly. Take all your clothes off. We will stop the traffic for 30 seconds. You will lie, heads together, in five separate star-shaped groups. Your hands must be by your side. Your eyes will be closed. I will get on to a ladder and take one roll of film. This will be fun, but please laugh afterwards. OK. Go! Go!"

We all storm out of the Holiday Inn. The models rip off their coats, shoes, jeans and shirts and dump them on the pavement. Fiona strides into the middle of Berkeley Street, stopping a bus and two cars. "Quick, quick," yells Spencer, hopping around with his ladder.

A man called Guy, wearing a silver-buttoned frock coat and cravat, is furiously playing the harmonica. Twenty-five nude people run into the middle of the road. A Japanese tourist stops and takes a photograph. Unfortunately, she is also in the middle of the road. "Get out!" screeches Spencer from the ladder. "Get out of the shot!"

The models squirm about a bit, settle down on their backs, then Spencer fires off a few shots from his ladder. Some of the men have erections - fairly uneventful ones, admittedly - but Spencer has already thought of this. "Flip, flip!" he shouts after a few seconds, and everyone turns, stomach down on to the tarmac. We are all slightly tense; if the police turn up, there may be problems. Spencer jokes about us visiting him in jail, but luckily, they don't appear to have been alerted.

The sense of vulnerability offered by the scene is overwhelming. Naked shoulders, arms, rounded buttocks and bony backs look soft and frail on the ground against the concrete and glass of the Mayfair office blocks which rise up around them. Buses cruise past behind the nude figures, appearing as monstrously dangerous machines. But just as you start to worry about a runaway No 38 piling into this soft mattress of geometrically arranged flesh, the shoot is over. "I got it, I got it," squawks Spencer, beaming. "Run, chickadees!"

Everyone leaps up off the road and storms back to the Holiday Inn to get dressed. Or rather, back to the pavement outside the Holiday Inn, whose staff are not amused by this harmless, rather joyful event."We were never told people would be naked! Outside our foyer!" says the doorman with fury. "You will not come back in." No one cares; we have all been involved in a far more interesting experience. The models hop around with cold, laughing. "I was in a puddle," says Colin. "But it was great."

Spencer surveys his models with pride. He is now wearing a pair of black wraparound sunglasses; people surround him, hanging on his words, hoping he'll go to breakfast with them. "All around the world, people who pose for me are the same," he says. "Bodies are the same everywhere. It's just the streets that are different."

Spencer Tunick's `Naked Pavement' show runs until 11 March at the Special Photographers Company, 21 Kensington Park Road, London W11 2EU.