Crammed into a cupboard-like changing room, 25 women jostle for space as they change into push-up bras, skimpy vests, hot pants and fishnets. They are not backstage at London fashion week, nor are they pole dancers (they would no doubt lamp you for the suggestion). This eye-popping gang are the London Rollergirls, and, just as soon as they have donned their helmets, pads and skates, they are set to play roller derby, one of the UK's fastest growing - and some say most feminist - grassroots sports.

The name might sound innocuous, but roller derby (rhymes with Furby) is a world away from what most British women associate with roller skating - those innocent evenings spent wobbling around the roller disco. An American contact sport, roller derby is based on formation roller skating around a track and involves a surprising degree of violence between teams, often including vicious elbowing (see box opposite for the full rules). And these women take their sport very seriously. When I call Belinda Carlson, a member of the London Rollergirls, she warns me in no uncertain terms that they "are not a novelty act".

The London Rollergirls formed in April last year, the first roller derby league in the UK. The evening training session that I attend takes place in a leisure centre in London's East End - one of only two sports venues in the capital with a space even vaguely big enough to accommodate the roller derby track.

There is a lot of shouting as the women warm up and call each other's names. Their skate names, that is: rather like in WWF wrestling, each has her own stage name, which she registers with an official database in the US. Every name has to be unique: Tear E Hatchet, Lauren LaBurn, Sleazy Rider ... If your chosen moniker is even vaguely like someone else's, you have to seek them out and get their permission before using it.

Watching from the wings today is Bruise Violet, aka April Lorren. She is not playing because she is on crutches after breaking a leg during practice a few weeks back, her injury a stark example of the sport's hazards. Lorren is a director of the London Rollergirls, and, as an accountant by trade, looks after the finances. The players themselves own and run the show, diverting any profits to women's charities.

"What I really love about roller derby is that you have a great support group of amazing women around you," says Lorren. "The kind of women who do this sport tend to be really strong ... And, although it's a generalisation, for the most part women with strong personalities don't get along with other women with strong personalities. But here we're thrown in together and have to get along."

Invented in Depression-era America as a spectator sport by businessman Leo Seltzer roller derby was originally played in mixed teams, but is now generally women-only. The sport's US heyday was in the 1950s and 60s, when games were regularly televised and players were paid professionals. It subsequently fell out of fashion, but has recently enjoyed a major revival as an all-girl amateur sport: there are now more than 135 American leagues.

Over the past 18 months, roller derby has also been gaining popularity in Europe. Leagues have sprung up in Germany and Sweden, and four have formed in Britain: two in London, one in Birmingham and another in Glasgow. This is not a sport intended to be played behind closed doors. On the contrary: these women want to attract paying customers. Wallflowers need not apply, because half of the appeal is the theatrics, wearing the raunchy uniforms and playing up to the (mixed) crowd.

The Rollergirls say that the game helps them to get out their aggression. "What happens on the track really stays on the track," says Bikini Killer, aka Jen, an American who used to skate with the Detroit Rollergirls. "Usually it's just a case of seeing someone at the bar afterwards and saying sorry if you did an illegal move, like giving them the elbow."

But another skater, Iva Issues, from the Birmingham Blitz Derby Dames league - aka 37-year-old cognitive hypnotherapist and mother of one Bee Bentley - admits that there can be conflict. "When you get a bunch of strong women together there is inevitably twisted derby drama - the squeakiest wheels have a tendency to swing things around in their favour," she says cryptically. She points out that it was a fight between some of the London Rollergirls that led to the creation of a rival league, the London Rockin' Rollers.

"I have a degree in psychology and it's a mind trip how these girls relate to each other," says Bentley, who used to skate with the Rocky Mountain Rollergirls in her native Denver, Colorado. The squabbling tends to reach its peak when a league splits up into different teams, who then become rivals, she says.

Watching the London Rollergirls' practice - in training for their first public bout on September 8 - it becomes clear there is a rivalry in the costume department too. The skater with the coolest helmet is 37-year-old illustrator and animator Emma Broughton, who is sporting what looks like a stainless steel fishbowl on her head and circular, thick-lensed glasses. It's a deranged look, but it works and only really becomes terrifying when you catch the glint in her eye as she admits it feels "fantastic" to knock her opponents over.

Also skating like the wind is Brogan Savage, a 23-year-old personal assistant from Essex who goes by the name Bambi Manslaughter ("it's inspired by the Essex mass murderer Jeremy Bamber. I wanted Bambi Manhater, but I thought that was a bit, you know ... "). She admits that, for her, the roller derby appeal was initially aesthetic.

"I had read about it in Bust, an American feminist magazine, and I loved the outfits and the way the girls looked. I couldn't even skate when I joined," she says. But she was soon hooked - despite some faintly horrific injuries, including tearing her knee ligament three times. "What I love is that it's a do-it-yourself thing, with women at the helm," she says.

Most skaters strongly assert the sport's feminist credentials as an activity that bucks notions of women as soft, gentle and nurturing, and instead allows them to express their aggression. "I think doing something and excelling in it and feeling great about yourself is very empowering," adds Anna Monoxide, whose thigh sports the first London Rollergirls tattoo - she's thinking of getting another on her labia. And she does not accept the idea that it is somehow anti- feminist to dress in saucy outfits and encourage men to pay to watch.

"We have the right to wear fishnets and short skirts if we want to. It's our own choice," she says. "I wear a short pleated skirt - with shorts underneath - because it gives me a lot of movement. You're going to have guys as well as girls watching and it is a spectator sport. But it is still a real, serious sport."

(And it is worth pointing out that dressing provocatively isn't a prerequisite. While the sport is associated with revealing punk outfits, the uniform worn by the Birmingham league, for example, has a far more military look.)

Though men are not allowed to skate, they are invited to pitch in, as roadies, cheerleaders, masseurs, medics, photographers, uniform suppliers and sponsors. The London Rollergirls' referee is Splint Eastwood, a towering man in a rather dashing kilt. He says he is not in it for titillation. "I like it because it's a unique sport. And anyway," he says, "having 20 women hate you kills the male gaze very rapidly."

Blocking and jamming

How to play roller derby
Roller derby rules vary from league to league, but generally the sport involves two teams of five skaters. All skating is performed counter-clockwise on a small, narrow track. Each team normally consists of four blockers and one jammer. The jammer's job is to break ahead of the pack and score points by doing laps, outskating the opposition blockers, who try to hamper their progress. Physical contact between players is frequent and sometimes violent. Body blocking is allowed, and elbowing is permitted in some leagues, but participants cannot trip or intentionally punch other players. Penalties are given to skaters who block illegally, fight or behave in an unsporting manner.

· To see an audioslide show of the London Rollergirls in action, visit theguardian.com/inpictures Team websites include www.londonrollergirls.com; www.myspace.com/blitzdames; www.myspace.com/glasgowrollergirls