I've got a funny-shaped body and I live in a world that is incredibly uptight about what constitutes normal or acceptable when it comes to bodies. It's easy to get the message that what I've got to offer, bodywise, isn't much of an offering at all. So I do what I can to reassure myself that I'm okay; it's a daily challenge.

I take any opportunity that enables me to see myself as part of the great spectrum of human body types: I hang out naked with my friends at the baths, I go dancing, I swim, I don't hide behind a towel in the changing rooms, I sunbathe in a bikini, I try not to judge, I watch people of every walk of life and look for the goodness in their bodies so that I may feel less damning of my own. In short, I try to be adventurous and unafraid.

A few weeks ago I saw an advert in 'Time Out' that rang my bell. It said: "BE CONSUMED Pose nude inside Selfridges for a photograph and installation by Spencer Tunick. Sunday April 27 2003. In exchange for posing you will receive a print of the event by the artist. You will only be nude for a short period of time. Wear loose fitting clothing and no jewellery. Leave your valuables at home. Bring friends and family."

I signed up for it, and persuaded Simon and Kay to take part too. Okay, I had no idea who Spencer Tunick is, or even how to say his name, and I thought the whole thing might just be an advertising thing for Selfridges, but I didn't care. The idea of being part of a naked crowd in the middle of a department store seemed like fun, I'd seen mass naked pictures before and thought they were great, kind of envied the participants for being so free with themselves, and so beautiful – and I'd get a picture of the event too.

A week before the gathering I got an email that spelled out the details. We'd be posing inside the shop early on Sunday morning before opening. It would be more-or-less a closed set with only those participating allowed to watch. We had to be over 18. Tunick added: "Thank you for wanting to be a part of my art! I could not make my work without you, so I am extremely grateful for your participation. This is going to be a great, beautiful, new experience!"

The morning of the shoot we stood outside Selfridges with 500+ participants, signing consent forms, being handed plastic bags in which to store our belongings, feeling excited about whatever might lie ahead, and generally checking each other out. A couple of hundred people were turned away because there were too many participants. Most people were white, aged between 25 and 40, kind of trendy, average-sized, some gay boys, noone I knew. We played "Who would you most/least like to see here?" (Simon: Tony Blair).

Once inside we sat on the floor on Women's Accessories, were given general instructions by Tunick, standing on a balcony with a megaphone, and then it was time to strip.

This was easily the best part of the experience. People were so excited they wasted no time in taking it off, after all, that's why we were there. Within ten seconds I saw tits, then arses, then dicks, then 500 naked people standing and blinking at each other. Everyone looked happy, there was a feeling of delight and discovery in the air, the scene was very surreal (yeah, I know, understatement). At first I didn't know where to look, and then I just looked, mostly at the willies, which were many and varied.

As people moved to take their position in the cosmetics hall, skin brushed against skin disconcertingly and I started to laugh at the loveliness of the scene. Everybody looked so great! We were being so brave and cool. We looked funny and incongruous amongst the make-up counters. I was really glad to be alive and participating in this strange experiment.

Tunick managed us from a set of portable steps at the end of the hall. He worked with a team of slick shoot managers. We stood, facing away from him. We collapsed on the floor. We gathered around the counters and stared blankly. We were a mass of naked flesh, and we looked as though we'd just been born, full-sized, and plonked in that bizarre environment. I could see pedestrians walking past the main doors outside. Few noticed us.



The next set-up was planned for the escalators. All 500 of us were herded onto the central escalators between three floors. Tunick wanted three people to each escalator step. It took a while for everyone to get in their place and as the escalator became more and more overcrowded with nudists I felt it shaking. I started to get a bad feeling. There was nothing beneath us and I wondered how many people the escalator was designed to hold safely. I remembered the consent form we signed, that included a waiver against any injury we might sustain during the shoot. I was sweating and scared, trying to hold onto the rail, stop imagining naked bodies falling through the atrium, and calm my breathing. The escalator began to lurch and then rolled backwards about ten feet, as though it was working without any power. People gasped. My life started to flash before my eyes. I was afraid we'd fall or get our feet tangled in the metal teeth. I was really panicking, I had to get off that thing. People stood at the bottom and stared, not quite believing what was happening. I pushed my way off and Kay was right behind me. We were shaking with fear. I saw Simon further up the escalator, kind of trapped in there. The shoot continued.

At this point the women were separated from the men. We went up to the third floor to pose in the lingerie department. It was weird walking past groups of men as we made our way up the escalators, like running the gauntlet in an "I've forgotten to put on my clothes for the first day of school" nightmare. I looked at the ground. Later on in the day there were cleaning staff getting the shop ready for opening. They stood and stared and it was fine, funny even, because we naked folk were all together in a group, staring back.

I had been feeling anxious since the escalator incident. My thighs were damp with what I assumed to be nervous sweat. I wiped them with my hand. Uh-oh. I'd started my period. Periods have a habit of turning up when you least expect them, usually in the middle of the night, staining your favourite sheets. One time I sneezed so violently that it set off a period prematurely. I wasn't due to start for another week but, between you and me, I think the shock of facing my mortality on that escalator in the nude was enough to get me bleeding. It truly could not have been a more inconvenient moment. I was naked, in the middle of Selfridges, about to pose for a photo on a pale carpet with a couple of hundred women. Jeeezus!

I ran for help. One of the shoot staff found a security guard who took me to the toilet, a tampon was found. Kay helped me clean up as much as possible and then we returned to the set. We sneaked into a corner and posed, although we knew we'd be out of shot. I didn't mind, I came for the experience.

It was really weird being separated from the rest of the group. After experiencing a safety in numbers I felt extremely self-conscious and vulnerable. The male security guard was great, very gentle and respectful, but we were in such a rush, worried about bleeding everywhere, scared of being left behind, or lost, that it was impossible to enjoy the strangeness of the situation.

Back downstairs we put on our clothes and watched the final set-up which involved the men tucked up like little pebbles. And then it was over. In the rush to clear up I saw my friend Patrick and met his boyfriend for the first time. Patrick said: "Don't look at my tackle!" which made me laugh. We agreed that it had been a great experience.

The transformation scene back into clothing was almost as strange as the mass undressing of earlier on. Nudity made people appear relatively anonymous and clothes gave everyone back their identities. At first I wondered who had let in all these outsiders to watch, but then I realised that they were my fellow models back in the real world of clothes. Although the flier promised that we'd only be nude for a short time, it was more like an hour. I was glad to be dressed again, the escalator fright and the onset of my period were just a bit too much for me to cope with whilst being naked!

During the shoot I got glimpses of what the pictures might end up looking like. Wave upon wave of blank-looking, anonymous, naked people (Tunick through his megaphone: "Don't smile! Don't look at the camera! Arms down!"), bodies in deeply absurd surroundings, the sheer scale of human variation. I will treasure the memory of seeing so many wandering around in the nude, looking up, walking, being with their loved ones, being good-natured and generous, acting simply. It was a splendid vision of humanity. Although the shoot had its ups and downs for me, I came away feeling very proud of myself for being brave, glad that I had taken part and that Tunick is making these amazing images, and satisfied that although I still look funny, it's fine by me.

PS Since writing this piece I've been getting occasional emails from nudists/naturists/naked people inviting me to various naked things. Whenever I've read the story in public, it often happens that a man sidles up to me afterwards and says: "so, you like getting naked do you?" Those naturists really love to recruit but, people, I am not a naturist, I just posed for a weird photgraph one time and I never want to do it again. Please, save your invitations and sidling for someone else.