Here, scruffily dressed, largely young and male free runners gather most weekends to practise their moves, compare techniques and generally roll and jump around like an army of stuntmen, albeit without the movie cameras. There is a film crew here this cold December weekend, however; from Brazil, for the prime-time Fantastico show.

Not all parkour devotees are young and male. Emily Rogers is an enthusiastic traceur, though she hardly fits the stereotype. A Cambridge University graduate in English and journalist by day, Rogers, at 27, is older than most parkour enthusiasts and soon to be married. She says she feels like 'the mum', although she is by all accounts rather good at the sport despite only a year's involvement.

She has just returned from a demonstration in Paris, for Renault. When I first call, however, this particular free runner is feeling far from free. 'Sorry, I can't talk. I'm in A&E [accident and emergency]. Can I call you back?'

'It's nothing too serious,' she says later, laughing. 'Just a fractured finger.' You'd think she had got off lightly, what with all the death-defying feats, but as with most traceurs, she is at pains to point out her favourite pastime is 'very safe'.

Rogers is a self-confessed tomboy who always wanted to bounce off walls and climb things. Her background in gymnastics taught her how to roll and tumble, giving her a head start in parkour. She stumbled upon parkour after watching last year's seminal television documentary Jump Britain, the sequel to Jump London from two years before, which showed traceurs, led by Frenchman Sebastien Foucan, scaling famous landmarks such as HMS Belfast and the Royal Albert Hall. Both documentaries followed a breathtaking BBC trailer entitled Rush Hour, in which acclaimed traceur David Belle leaps and runs across London via any means but terra firma; across the skylines, jumping roofs, walls and ledges, to get home in time for his favourite television show. An internet buzz followed and the 'sport' took off, fuelled largely, it is said, by agile young men bored of skateboarding.

Belle is the acknowledged grandmaster and guru, says Rogers, the star of many films and adverts.

Parkour is essentially a form of movement, derived from the parcours du combatant; the French military obstacle course. It is defined, if you can define it, 'as a quasi-sport in which participants attempt to clear all obstacles in their path in the most fluid manner possible'. Immortalised in Luc Besson's cult film Yamakasi, in which traceurs are portrayed as modern-day samurai, the sport will be seen by the world at large in the new Bond flick, Casino Royale.

'David's dad was a military firefighter, a great athlete, who worked on ways to escape certain situations in emergencies,' says Rogers. 'He drilled moves such as vaulting, jumping, running, swinging. None of the moves need to be used in everyday life, but that's the point, we can move like that if we want, anywhere we want. It's the intention. Moving like this, especially within the confines of an urban environment with all its obstacles, gives off a wonderful sense of freedom. Perhaps the first thing you learn from serious free runners is that it is anything but an extreme sport and that few practise the infamous jumps off buildings.'

EZ, 31, a former boxer and martial-arts enthusiast, is a leading light within Urban Freeflow, a collective of performers and enthusiasts, and perhaps Britain's most famous parkour group. The group makes a living out of the pastime; touring the world giving demos, classes and now, given the commercial possibilities of the mushrooming sport, selling T-shirts and trackpants through its website. EZ has just returned from a tour of the United States and Japan, and is off to France next week. This weekend, however, he will be somewhere less glamorous: Cardiff, Wales, for some safety demonstrations.

'It is not an extreme sport. No way,' says EZ. 'It is more a way of life. It's pushing a sense of freedom of movement; that urban obstacles needn't be obstacles. It's all about the art of movement, using objects within an urban environment; running, vaulting, spinning, doing tumbles, a whole range of manoeuvres - rolling on concrete, clearing barriers or swinging on bars.

'But the biggest achievement is to move fluidly and emulate the movements of animals like cats and monkeys. It's not about doing 'tricks', it's more about linking techniques together seamlessly to simply create flowing movement.' One jump does not make parkour, but putting three together, in a fluid, seamless movement over obstacles, does.

'When performing, we take calculated risks; it is by no means gung-ho,' says EZ. The areas our crew use for many of our moves are checked and measured beforehand, much like a stunt co-ordinator would do in films. We are very safety conscious. I have been doing it for three years now and only picked up minor injuries. It's almost street gymnastics, but with a spiritual side.'

If it is so safe, why has EZ just come out of hospital after a knee operation? A bad fall from a perilously high building? 'No. Just wear and tear really. Just overexertion on the tendon, spread over time. In basketball, they call it 'jumper's knee'.

'You have to realise the building jumps are rare, often just for the cameras. It provides a wow factor. It gives the sport some media coverage, but in most of those jumps, we use nets and wires to be safe. Fluidity of movement is the main aim. Almost like moving to rhythm but without the music.

'By doing Parkour, you become more in tune with your surroundings and develop an appreciation of architecture and how to interact with it. On a personal level, you become more aware of your own body and its capabilities. This leads to a rise in confidence, at least in the sense of posture and how you carry yourself. This can carry over into all forms of sport and general life.'

Does Rogers think parkour an extreme sport? 'It's semantics really. Some say it is, some don't. Surfing is dangerous, but people don't call it an extreme sport. It can be dangerous if not done properly, as can skateboarding and Rollerblading.'

Most crews are keen to promote the safety aspect, especially after one British 14-year-old novice slipped and fell to his death last August. EZ says the sport grew phenomenally after the Jump Britain documentary, 'which shot people jumping off car parks etc. We had all sorts of tales, with kids coming on our message boards saying, 'I hurt my ankle and I only jumped off a four-metre wall.' We thought, 'God! We'd better do something here,' so we started hosting workshops to spread the safety message.'

Rogers says roof jumps receive a disproportionate amount of attention because of their nature; they make for an extreme photograph and create a spectacle not available on ground level. 'But when you see a picture of a roof jump, you rarely see that the jump is actually quite low. The thrill is not the height, or the danger, but getting the technique right.

'Also you have to think before you do it. There is a belief side to this. A bit like martial arts. If you want to run up a wall, you can. Don't just say, 'I'll get to that point', say, 'I'll get over that wall'. It's the antithesis to what your mum always taught you. Kids are told, 'Don't walk there, come down off that wall.' We are taught that walking on a high wall is dangerous, but it needn't be if you do it often enough. It's good for children and adults to learn to run free, explore a sense of freedom.'

Parkour is growing rapidly. Urban Freeflow has 13,000 message board members and its site scores about 180,000 hits a day.

'I guess Britain is the hub of the sport now, although it started in France. But its pull lies in its accessibility. You can do this anywhere, of course, not just in the urban environment, but cities are crowded, chaotic places, with lots of barriers, a place where you feel more trapped and thus somewhere where parkour makes you feel less trapped,' says Rogers, whose own collective is called worldwidejam.

The spectator appeal in parkour lies in watching the skilled perform moves such as the 'blind jump', a precision leap undertaken from a spot where you can't see where you are landing; the 'tic-tac', in which your foot grips a small hurdle to propel yourself more easily over the next obstacle; and the 'cat jump', in which a practitioner places both hands on a wall and leaps through them with the legs moving through the middle of the arms. The art or technique is in landing much like a gymnast; safe and steady.

Rogers must have landed none too safely to hurt herself? 'We were doing a show in Paris,' she says. 'It wasn't just parkour, but all manner of acrobatics and jumps. Let's just say we mistimed the back flips. It was a minor clip, no one in the audience noticed, but I fractured a bone in my finger. It's funny though, I've never been hurt before. This is my first proper injury.'

People at her workplace might not see it as funny. She is having to type a story with one arm in a sling: 'I can't press alt-delete, unless I use my tongue.' Maybe she has spawned a new sport. Free typing anyone?