As Lot 173 was called at Sotheby’s Contemporary Art sale 10 days ago, a hum of recognition rose from the sleek, polyglot crowd. They knew they had seen the image — an aggressive young black man aiming a CCTV camera like a rifle, straight at the viewer — but where? In fact, Ladj Ly, by the French artist known as JR, had been plastered 100ft high on Tate Modern’s exterior last year. The closed-circuit gun had fixed St Paul’s in its sights. After that exhibition, the Lazarides Gallery bought a smaller version of the image from the artist, for £4,000. It appeared at Sotheby’s with an estimate of £10,000, and sold, to a French telephone bidder, for £26,250.

In a modern art market where Hirst has burst, and nobody, at present, knows anything, this is more than a decent return. The sale is testimony to the way in which JR has caught the mood. Collected by established dealers, first-time buyers and the gossip-column rich, he has become, in the words of Isabelle Paagman, Sotheby’s head of contemporary art, “unbelievably hot”. Indeed, Trudie Styler (Mrs Sting to you and me) has bought one of the other four existing prints of Ladj Ly. She likes it that you “can’t pin JR down . . . like a lot of great artists, he’s enigmatic”.

One can pin him down this far. JR is a 25-year-old Parisian of mixed race (he has Tunisian and eastern European blood), from a middle-class background. He never reveals his full name because it “would add nothing”. In his teens, he “tagged” as a graffiti artist, but only started taking photographs when he found a camera on the Paris Métro as a 17-year-old. He is now, in his own words, a hybrid “photograffeur”, who pastes enormous black-and-white photographic canvases in various urban environments. In Britain, his best- known work is certainly Ladj Ly — a photograph that captures all the tensions of the 2004 Paris suburb riots. But he has made his mark around the world.

JR has, for instance, just returned from Kenya, where he and his band of 10 or so unpaid assistants turned Kibera, one of Africa’s biggest slums, into a vast exhibition space. Last year, JR had visited Kibera to take photographs of its residents. He returned this year, after the mayhem of the election riots, to plaster their portraits on train carriages and on the roofs of their houses.

By using waterproof vinyl material, he ensured his art might have a practical purpose. “The more you go to places like Kibera, the more you realise that the people don’t understand you,” he says, in his faultless, breathless English. “Food is their first need. They don’t do art just for the love of art. It has to make sense. By making their roofs rainproof, what we did made sense. They loved it.”

The exhibition also looks beautiful. JR has a thing for eyes and noses and mouths. Viewed from above, the effect is a kind of physiognomical carnival — one that emphasises the vitality of the photographic subjects. “Normally,” says JR, “you have to be very famous to have your picture blown up so big. But these are just ordinary people, with everyday stories.”

JR’s work in Kenya is only the latest in a number of ambitious community projects he has undertaken to aggrandise “ordinary people”. Last year, he visited a notoriously drug-ridden favela in Rio, where, despite fears for his safety, he managed to take portraits and paste the images of women and children onto the tin walls. The results were mesmerising: a shanty-town on a hill transformed into a living collage.

Why does he choose the planet’s poorest places for his “actions”?

“I see them in the media,” he says. “But I want to see them with my own eyes. You realise when you do go to these places that there is no art. My aim is to show that art can work anywhere.

“The favela where I went is right in the centre of Rio. There are no social institutions there, no NGOs. It’s all owned by the traffickers. It’s the worst place you can imagine. But they have just accepted to have the biggest exhibition in the world on their houses. By just doing that, you are showing that things are possible . . . although I would never say I am changing the world.

When I went back to Rio, one month after the project, a lot of the kids who’d helped me were back dealing drugs.”

JR says he is not political. Rather than being an “artist with a cause”, he is “an artist who causes people to think”. He has never caused more ructions than he did in Israel and Palestine, where he pasted photographs of three religious worthies — a rabbi, an imam and a priest — pulling silly faces. JR put their pictures everywhere: in Ramallah, in Tel Aviv and, most famously, on the wall that separates Israel from the West Bank. He was arrested by the Israeli army for his trouble-making.

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