Sao Paulo, Brazil (CNN) -- For many it's the adrenaline, the thrill of scaling buildings to scrawl their gang's signature in huge black letters from the highest point possible.
For some, it's also political -- a way to leave their mark on a city they feel has turned its back on them.
They are Sao Paulo's pixadores or taggers, marginalized youth who prowl the streets late at night armed with paint cans and rollers.
They have defaced buildings with Celtic-looking lettering across downtown and along many of the city's main roads.
"Where I come from, the only way to get famous is being a professional football player or drug dealer," says Cripta Djan, one of the city's more outspoken pixadores and gang leaders. "The other way is with pixacao."
Just don't call it graffiti. Djan says pixadores don't want to be pigeonholed as graffiti artists and aren't interested in beauty.
Instead, he says they are rebels who want to defy authority and break the law.
"It's class warfare," he says.
"Our society is very capitalist. You're only worth what you own. With pixacao you can invert the values. You don't have to have money to be recognized. You just need paint and you can write your name all over the city."
Pixadores live in poor suburbs but their preferred target is seedy, downtown Sao Paulo, where visibility is guaranteed. They say that in the wealthy neighborhoods their signatures would disappear in a day.
They leave their mark on bridges and underpasses, invade abandoned buildings to gain access to the top floors and scale skyscrapers by climbing from one window frame to another.
Some have fallen and died.
Pixacao emerged in the 1980s in Sao Paulo, a sprawling city of 20 million people where the division between rich and poor is vast. Pixacao has since spread to other cities in Brazil.
Well-known pixadores like Djan have participated in international art events but they invariably end up sparring with organizers.
"If there's no transgression, it's not pixacao," says Cripta, who recently defaced a church in Berlin when he was invited to give a workshop at the Bienal there.
While pixadores have fascinated art afficiandos, they don't necessarily have many fans among the rest of Sao Paulo's population.
"Those guys are vandals," an office worker says. "We were better off under the dictatorship when at least a minimum of order was maintained."
"We should have harsher laws and better education," another woman says.
On Thursday nights, hundreds of pixadores converge on a street in the center to brag about their latest feats and plan their next targets as they drink beer and smoke cigarettes and marijuana.
They spray the names of their gangs on walls and compose rap songs, about everything from Brazil's consumer society to Bob Marley.
They are mostly young men, but a few women are also active pixadores. A young mother and student, Michelle says she formed a gang with eight other women.
"It's adrenaline, art, freedom of expression," she says. "You have an idea and you want to rebel."