A dip at dawn, a poolside party - and an oasis of calm in the frenetic city

As in life, there's a shallow end, a deep end and, in between, sporadic shrieking, laughter and bare flesh. Unlike life, there's also a little Art Deco pit stop, and a sign proclaiming Brixton Beach Café. This is a fib, geographically speaking - we're actually a couple of miles from Brixton - but Brockwell Lido is an urban beach, in spirit at least, and among the last of its kind.

Opened in 1937, the lido was a product of the rare British bout of civic-minded planning that saw hundreds of similar outdoor pools built in the first half of the 20th century. In their heyday, the 1930s, they offered the masses sun, swimming and the open air, for a penny or three a day. Now only a fraction are still open in anything like their original state.

The greatest number closed in the 1980s and early 1990s, sometimes replaced by claustrophobic leisure centres where the weather isn't an issue but the atmosphere is unbearably stuffy. "In a country where the average temperature hovers around 17C nothing testifies to the optimistic spirit of interwar Britain like a lido," says the Twentieth Century Society, which campaigns for the listing of endangered pools, including those in Scarborough and Plymouth. "These outdoor, often saltwater, baths are more than romantic reminders of plucky Britons in knee-length togs. Many of them are also fine examples of the architecture of leisure, often characterised by a delightful mixture of Art Deco and the International style."

The lido was the creation of the architects H A Rowbotham and T L Smithson. Nearly 70 years on, up to 2,000 people, on very hot days, are still thanking the pair. Like so many of the few remaining lidos, Brockwell has survived thanks to the dedication of enthusiasts. In 1994 the near derelict, squatted wreck was taken on by Paddy Castledine and Casey McGlue, both 39, who still run it on a short lease from the local council. Thanks to them the pool has been saved, from the bulldozer at least, and last year this low-rise, ivy-clad, largely intact gem becameGrade II-listed.

"I've been coming since I was a kid, before the war," says Alf O'Connell, a retired council worker. "People used to pay threepence to watch the diving from a spectators' enclosure. Uniformed blokes kept an eye on any bad behaviour. They'd give you what for and clip your ear." Stretched out on a deck chair is his friend Paul Kirwan, 70. "I first came here in 1971. It's peaceful. I like the place just as it is."

And it is a unique place. For London, it feels disconcertingly safe - but there are no guards, security cameras or metal detectors at the gate; there is no "gate". You just walk in. And when you've walked in and found a lifeguard to pay you join a couple of dozen people relaxing, chatting quietly, staring at the sky, reading, smiling, and moving at one-quarter the speed normally demanded by inner-city living. And you do so unassailed by advertising, sponsorship-boasting posters, or mission statements.

But change is imminent. Usually, the lido makes an annual operating loss of between £150,000 and £200,000. For several years it has operated without subsidy from the local council, Lambeth. After a report by a leisure industry consultant and talks with local groups, the council has awarded a 25-year lease to Fusion - a "motivated" non-profit organisation that has built a reputation as an efficient operator. And it likes mission statements. When Fusion takes over the lido next month a clash of cultures seems inevitable.

"We've created a special place here," says Casey McGlue. "We're laid-back, and some people say that's not the way - but it works." His business partner and lido co-saviour Paddy Castledine is equally wary of the future. "I'm hoping that Fusion don't do the Big Brother act. Last year we had 15,000 people through. It doesn't sound all that much, but this was all races, creeds, classes, whatever - and not one bit of trouble. Everyone left smiling."

Mary Hill, who runs the Brockwell Lido Users group, sees the Fusion option as the only viable way forward but is keen to preserve the pool's special peace. "There is a particular ambience here. It's very friendly, relaxed, people feel safe, women are happy to sunbathe. The people running it understand the community. But it does need someone who can put investment in."

Fusion has pledged £2m to the pool and its buildings, and says some will be "sensitively" extended to accommodate a greater number of non-swimming activities such as yoga. But will this still be the egalitarian, restful pool envisioned by Rowbotham and Smithson? Mr Castledine is wary: "Lidos are part of nature now. If you mess about with nature bad things happen. The people who built these pools knew what they were doing."

THE VETERAN SWIMMER: Alf O'Connell

"I've been coming since I was a kid, before the war. I come as often as possible, and sometimes I get here at 7am. There used to be a diving board, and blokes in uniform keeping order. People were rowdier then. Things are a lot easier here now."

THE CAMPAIGNER: Mary Hill

"This pool has gone from crisis to crisis," says Mary, chair of the pool users' group. "All these 1930s buildings need investment. I'm here just about every day. Someone can hand their baby to someone else while they swim - where else would you get that?"

THE MANAGER: Casey McGlue

"We have let in about 2,000 people on very hot days. We get mobbed. We've even had 400 people queuing outside. This is a very special place - we run things differently here."

THE ENTHUSIAST: Paddy Castledine

"I'm a local boy," says Paddy, Casey McGlue's co-manager. "I've been coming here since I was three. When I was a kid, I used to think: 'Wow, what would I do if I had this place.' It was closed and totally derelict when Lambeth council gave us the keys."

THE LIFEGUARD: Mark Davey

"We're here during the summer, and we live on site," says Mark, an 18-year-old student. "It's perfect here, such a great place to chill. Good company. I come on duty at 8am and finish about 6pm. On busy days it gets hectic. I am worried about the future of the pool."

THE FAMILY: Dalziel Douglas

"Sometimes I come in at 6.30am before the kids get up," says the café worker from south London, pictured with her children Mimi, 7, and Beatrice, 18 months. "I do 20 to 30 lengths. It's nice to be away from outside. The parties are great too."