PARIS: In the summer of 1998, Valentin Bechade, a young French graffiti artist who goes by the tag name Teurk, clambered onto the ruins of an old bridge and set to work with an aerosol can.

Graffiti artists are not usually popular when they spray-paint ancient monuments. Then again, this was no ordinary bridge. This was the ruin of the 16th-century Ottoman bridge of Mostar, destroyed by a Croatian tank bombardment in 1993 – since reconstructed in an iconic act of healing after the devastating Bosnian civil war, but then still on the front line of a bitter ethnic divide.

For Teurk, and a group of antiwar activists he had linked up with, painting the bridge was a political statement, an affirmation of creative power over vandalism.

“We realized this was a symbolic place,” he said in a recent interview. “We got the kids involved. Local people thought it was fantastic.”

Frank Esselink, author of “Vandals and Crusaders,” a history of graffiti, says that sums up the spirit of street art.

“We have to get rid of the idea that graffiti is a form of vandalism,” Esselink said. “Graffiti artists are crusaders.”

Ten years ago, graffiti art was an underground activity. Teurk was on the bridge in Mostar as a result of having been arrested by the French police, spray-painting a Paris freeway bridge. A community service order brought him into contact with Guernica, an activist group in Toulouse that had the bright idea of buying a bus and driving to the former Yugoslavia to put on an informal art festival for children. Like hip-hop and Travellers, graffiti was counterculture.

No longer. Last week, the Tate Modern, housed in a former power plant on the Thames, brought six street artists together from around the world to spray-paint giant murals on the museum’s river facade, kicking off the first major public museum display of street art in London.

In February, “Moona Lisa,” a stencil by the British street artist Nick Walker, depicting Leonardo da Vinci’s enigmatic beauty baring her bottom, sold for £54,000, or $106,000, at a Bonhams London contemporary art auction, more than 10 times the top end of its preauction estimate; and this month a bronze rat by Banksy, another British street artist, fetched $169,000 at a Sotheby’s contemporary art sale in New York.

Once ephemeral – scrawled on walls and subway trains, to be dissolved away by the cleaning crews – graffiti and its street art cousins have gone mainstream with a vengeance.

“This art is here to stay,” said Gareth Williams, a specialist in contemporary art at Bonhams London, who is now preparing his next urban art sale for Oct. 22.

To accompany the Tate Modern show, its curator, Cedar Lewinsohn, has published the first in-depth analysis of the phenomenon, “Street Art – The Graffiti Revolution,” tracing a history of the movement from neolithic cave paintings to the present day.

Each country’s street art reflects its own culture, Lewinsohn said in an interview.

The British sense of wacky, tongue-in-cheek irreverence comes out in works like “Christ with Shopping Bags,” by Banksy – a stencil of the crucifixion, with Christ holding a clutch of shopping bags in each nailed hand.

“Brazil is chilled out, relaxed,” Lewinsohn said. “For instance, Nunca uses ocher colors, influenced by tribal woodcuts and etchings.”

Nunca, one of the six artists chosen to paint the Tate river wall, started writing graffiti on the walls of São Paulo at the age of 12 and is now known for spray-painting giant faces of Brazil’s indigenous Indians in highway underpasses.

Parisian street art tends toward the cerebral, intellectual and philosophical. “Ogress,” by Teurk, shows an elegantly coifed and manicured young woman spooning cement blocks into her mouth – a confrontation between chic fashion and urban jungle. “She’s eating it like a strawberry,” Teurk said.

Miss Tic, one of the best known Parisian street artists, mixes philosophical quips with stenciled images of young women. In a typical example, a scantily clad girl, seen from the back, is set in counterpoint to the message: “Je ne croyais à rien mais j’y crois plus” (I used to believe in nothing but I don’t anymore).

Henk Pijnenburg, a Dutch collector who started buying street art 25 years ago, says that to fully appreciate the current wave it is essential to know and understand the work of the graffiti artists, mostly young black men, who started tagging in the New York subway in the late 1970s and early ’80s.

Artists such as Rammellzee, Dondi, Phase 2, Noc 167, Toxic, Seen and Quik, were politically engaged, warriors against the injustice of white society, their tag names a triumphant reassertion of suppressed identity, he said. Their work was often aimed at turning decayed urban spaces into people-friendly, colorful places.

“This is the most culturally significant art movement of the second half of the 20th century, since Cubism and Surrealism,” Pijnenburg said. “This art is very powerful, engaged and direct. After all these years I am still moved by its strength, and passion.”

The grounding in social conscience informs the work and the life of many of today’s artists. Toxic, one of the original New York subway graffiti writers, spends much of his time working with children, including two mural projects with high schoolers in Calabria and Tuscany, Italy, where he now lives. He currently is working on a project for the Red Cross.

L’Atlas, who, like Teurk, is part of La Forge, a studio and artists’ commune in the culturally mixed, hardscrabble Belleville area of northeast Paris, is also working with local children, preparing a giant compass-maze that will appear, and disappear again Sunday, as a one-day-only performance installation on the forecourt of the Pompidou Center contemporary art museum.

While some stars of the early movement, like Jean-Michel Basquiat and the white subway and street painter Keith Haring, won fast-track celebrity and critical acclaim, many others have been insufficiently appreciated, Pijnenburg said.

But that is changing. At a February sale by Artcurial, a fast-growing Paris auction house, a 1988 acrylic and aerosol work, “Call card” by John Matos, known as Crash, one of the original New York subway graffiti artists, sold for £43,372 – about $68,000 – a record for the artist and more than double the £15,000 to £20,000 estimate.

The entire sale of 276 lots brought in a total of £461,000, modest by the standards of the current contemporary art boom but, said Arnaud Olivier, a contemporary art specialist at Artcurial, proof of a growing market recognition of graffiti and post graffiti art.

“This art is new and fresh, and has brought a lot more players into the auction houses,” Olivier said.

This democratization of the market is an extension of the core politics of street art.

Haring, chalking his drawings in the subway, saw himself as bringing art to the people, according to Lewinsohn, quoting the New York art dealer Tony Shafrazi.

“Twenty million people travelling through the subway got to see his work,” Lewinsohn quoted Shafrazi as saying. “Keith considered that world to be almost a museum of his own kind.

“He thought that many of those people didn’t have the means or the knowledge to go museums, so he was bringing the art to them.”