It is one of the hottest days of the summer and outside London's South Bank Centre Jake Penny, 15, and his friend, Joe Scandrett, 13, are hurling themselves around a warren of concrete pedestrian underpasses.
Using ledges, metal handrails and anything else within reach, the two teenagers cause passers-by to gape by executing back-flips and somersaults high into the air, each time landing gracefully and silently on the pavement.
The chosen area for their exertions, less than five minutes' walk from Waterloo station, is a magnet for practitioners of free running, an acrobatic sport-cum-performance art played out on everyday obstacles - railings, walls, pillars, ramps, steps - that is rapidly moving from underground cult to mainstream phenomenon. Practitioners compete to run, jump and flip over and around urban obstacles as smoothly, elegantly and impressively as possible.
Having already won over the Royal Marines and James Bond, free running is booming this summer among young Britons. In response to the trend, councils have started looking at ways to take the activity into schools or even to create 'free run parks', similar to those used by skateboarders. Major companies, meanwhile, are pushing to have their brands associated with the increasingly fashionable pursuit.
This weekend the Sony Ericsson 'Off The Wall' tour reached Birmingham, after drawing big crowds in Liverpool last week. But free running will truly come of age next month when devotees come together in London for the first world championships, attracting major sponsorship from Barclaycard.
The competition is likely to provide fresh inspiration for free runners at street level, ranging from teenagers who dislike traditional team sports to businessmen bored with the gym. Jake, from outside Guildford, Surrey, who edges more towards tricks such as handstands and various types of flips rather than the 'running' aspect of the sport, said nonchalantly: 'I got into it from watching clips on YouTube and learning more about it on the internet. I used to mess around on my trampoline and then I just started going around to the gym.'
From there it was a logical progression to using outdoor street furniture. Both he and Joe have caught the attention of wealthy corporate interests, although for now their youth is a barrier to the lucrative earning power of their skills. But they resist the idea of their edgy pastime becoming an establishment sport, like skateboarding before it. 'I don't think I would do it if it really took off,' said short but powerfully built Joe, who, like Jake, favours wearing a simple T-shirt, baggy tracksuit bottoms and a pair of £10 trainers for the sport. 'Part of the thrill of doing it is knowing that you are not really supposed to be doing it.'
The activity derives most of its moves, style and philosophy from the Parkour tradition which originated in Paris in the Nineties and involves getting between two points as efficiently as possible by using nothing but the human body. Free running, which places greater emphasis on artistry and freedom of expression, took off in Britain after the documentaries Jump London and Jump Britain showcased the athletes against some of the most spectacular backdrops in the country. It has also been featured in a BBC 'ident' and several Hollywood films including Casino Royale and The Bourne Ultimatum, although Daniel Craig and Matt Damon's roof-hopping stunts bear little relation to the daily reality. Freerunning network Urban Freeflow has provided training to the Metropolitan Police and Royal Marine commandos.
Looking on approvingly last week as Jake and Joe pulled off ever more impressive leaps and jumps was Franck 'Cali' Nelle, a French former taekwondo practitioner. He began free running after coming to London more than a year ago and is now a member of Urban Freeflow, a five-year-old collective that has found lucrative work as a consultancy and stunt coordinator for films and TV adverts, as well as visiting schools to preach the gospel of free running.
Cali said: 'The reception we get in schools is incredible, even from kids who might not be interested in sports. Sometimes there can be a bit of resistance from parents when they hear what their children might be doing, but they come round when they realise that the skills and techniques of free running can be of great use in life. In other words we all come across obstacles in life, but those obstacles to what you want are not so great once you take a step back and work out a way of getting over them in a controlled and efficient way.'
Urban Freeflow is organising the world championships, which take place at the Roundhouse, north London, on 3 September. Twenty-five star performers from America, Australia, Brazil, Mexico, Russia, South Africa, the Caribbean and countries across Europe will take part on a specially designed course inside the cavernous venue, accompanied by their choice of music, then judge each other's performances for artistic merit.
Urban Freeflow estimates there are around 15,000 people who practise free running in Britain and that 98 per cent are male, with the internet playing a large part in its spreading the craze. It runs an academy in Kensal Rise, north London, where twice a week around 50 people, ranging from eight-year-olds to 55-year-olds, each pay £5 for two hours of tuition. Thirty free runners will show off their skills and host workshops at a special structure at the Mayor's Thames Festival near the London Eye in September. Newcastle City Council is hoping to arrange courses for young people across the city after the success of a two-day taster session in March, and Wrexham council has became the latest local authority to express interest in taking free running into schools after a campaign by local pupils. A dedicated parkour site has been proposed in Westminster.
Not everyone is so supportive, however, pointing to a lack of regulation and formal coaching qualifications. Alarms have been raised when videos turned up on YouTube showing young people leaping on roofs, trespassing or putting themselves in danger. The Safer Guildford Partnership, a coalition including the borough council, Surrey police and the Youth Justice Service, discourages free running after incidents in which people were swinging across a local river using a road bridge, then crossing three lanes of traffic. Tina Griffiths, a community safety sergeant, said: 'We are not trying to ruin people's fun, but this is a really dangerous activity, which could potentially cause injury to innocent passers-by as well as those who take part. We have already spoken to a number of people involved in parkour and given them advice about moving on.'
Scotland Yard and the London Ambulance Service said last week they were unaware of any incidents connected to free running. The Royal Society for the Prevention of Accidents urged free runners to be aware of their limits but said many sports can cause injuries and it has 'nothing against' the activity. Andrew Smith, 46, owner of Worldwide Jam, an international parkour and free running organisation, said that after 45 workshops in sports centres and schools involving up to 50 people each, it has seen a total of one broken wrist, one sprained shoulder, one pulled elbow muscle and 'a lot of blisters'.
Urban Freeflow director, Paul Corkery, 34, added: 'To be any good you need to be disciplined and focused. People who do it are sensible and serious and there's no unnecessary risk-taking. We do roof jumping for commercials, but always have safety nets and mats and it's all very controlled. Day-to-day training is at ground level.'
And what is the appeal of this peculiar sport in the urban jungle? The former boxer explained: 'It's absolute freedom of movement, being able to use your body to go anywhere any time. You don't have to spend any money, you just need a pair of trainers and your imagination.'