They defy gravity and surmount walls as would a movie martial-arts master. They run and jump, roof to roof, like Spiderman, seemingly oblivious to the distance between and the perils below. They wear neither helmets nor pads but vault over ledges and barriers seemingly at random, like stuntmen. They nimbly walk atop railings like gymnasts or tumble and roll neatly onto concrete floors like acrobats.
Welcome to parkour, or free running, a growing street phenomenon as hard to pigeonhole as its jaw-dropping feats are to fathom. Taken from the French parcour, meaning obstacle course, it is not exactly a sport; there are no competitions. Although it blends movement and mind, neither is it a martial art; its urbanity and daredevil antics make it to tai chi what skateboarding is to chess. Parkour is both a discipline and an art form, one combining aspects of gymnastics, athletics and dance. It is also every parents worst nightmare: it is highly exciting and very accessible because devotees, also known as 'traceurs' (or tracer bullets), don't need expensive equipment; just a pair of trainers, a free mind and an outdoor space chock-full of obstacles.
Parkour also needs a lot of courage and the ability to ignore pain, judging by the activities of scores of weekend free-running enthusiasts gathered at London's South Bank arts complex - a concrete shrine to 1960s modernism and now a temple for British-based free runners - where myriad alleyways, subways, walls, sculptures, statues, railings, steps, undulating concrete slopes, dips and banks are a playground for the more common urban tribes such as skateboarders and Rollerbladers.
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?'