SAO PAULO, Brazil » This mega-city’s authorities have waged war for years against what they call "visual pollution," banning billboard advertising, demolishing abandoned skyscrapers and planning to raze concrete eyesores like the elevated highway known as the Big Worm.

But the battle to clean up the sprawling cityscape has become intertwined with a deeper social conflict between Brazil’s haves and have-nots, where the angry and disenfranchised lash out in a form of expression unrivaled in other cities.

Taking action against the establishment, young people arm themselves with black paint, rollers, spray cans and no shortage of personal daring. Their target: the landscape that society cares so much to recover.

"We practice class warfare, and there are casualties in war," said Rafael Guedes Augustaitiz, 27. "They compare us to barbarians, and there may be a little truth in that."

Augustaitiz is part of a subculture that executes a form of graffiti described by one scholar as an "alphabet designed for urban invasion." It nearly envelops some of Sao Paulo’s government buildings, residential high-rises, even public monuments, with lettering eerily reminiscent of Scandinavia’s ancient runic writing.

The most daring practitioners risk their lives, scaling building facades at night to paint their script at the crests of smog-darkened skyscrapers. Some have fallen to their death from terrifying heights.

Their graffiti, called pichacao, from the Portuguese verb "pichar," or cover with tar, reflects the urban decay and deep class divisions that still define much of Sao Paulo, a city with a metropolitan population approaching 20 million. It is just one reminder of the social ills that Brazil’s economic boom has so far failed to resolve, and may perhaps even be accentuating, despite recent strides in reducing income inequality.

This month, Brazilians were stunned by clashes in Sao Jose dos Campos, an industrial city 50 miles from Sao Paulo, when the police stormed a squatter settlement, expelling about 6,000 residents from the area.

Fury over that episode and the violent clearing by security forces this month of crack addicts from a part of downtown Sao Paulo called Cracolandia produced a fierce protest last week against the city’s mayor, Gilberto Kassab. His car was pelted with eggs as he fled.

"It’s positive to see others reacting with indignation against our elite," said Djan Ivson Silva, 27, a pichacao gang leader. "We take our risks to remind society that this city is a visual aggression to begin with, and hostile to anyone who is not rich."

Often applying black paint with rollers instead of using costlier spray paint, the graffitists were influenced by the record sleeves of foreign bands like Iron Maiden and AC/DC, themselves influenced by gothic lettering and Viking runes, said Francois Chastanet, a French scholar and author of a book on pichacao.

The result was coded writing with vertically tilted black letters, often indecipherable to nonpractitioners. Chastanet says he marvels at the capacity of such an illegal lettering system to eventually occupy such vast swaths of a metropolis.

"For residents of Sao Paulo, it may contribute to blight, but we have to see that in its massiveness it is an urban wonder," Chastanet said.

Pichacao gangs often consist of about 10 members, mostly young men from Sao Paulo’s poor periphery, who paint short phrases, like "Poetic Terrorism," or their own names, like "Ze." Their tags rarely carry explicit political statements. Sometimes the painters just scrawl the name of their gang, like "Crypt." These groupings actually organize in broader associations called labels, which can encompass as many as 50 different gangs.

The labels, with names like "The Filthiest Ones" or "Registered Under the Penal Code," compete against one another to paint coveted buildings. Their street brawls are violent and can result in deaths. Such wars, as they are called by those who engage in them, can last years.

Pichacao gangs do not consider themselves graffitists at all, since colorful graffiti — in their view at least — is a lesser form of expression, easy to do on street level and often co-opted by the commercial art scene.

Some in the art world here find it hard to grasp pichacao’s appeal, especially after gangs gained prominence when they stormed into the Sao Paulo Art Biennial and Choque Cultural, a prominent gallery for streets artists, and defaced original works.

Other critics of pichacao question whether the practice is as politicized as some gang leaders say it is, or rather an empty form of expression simply degrading the city, instead of exploring new ways of improving it.

But in a development that would shock many Paulistanos, and even some in the pichacao scene itself, the foreign art world is starting to embrace the practice. One gang here has even been invited to attend the Berlin Biennale for Contemporary Art.

That is too much for Luiz Henrique do Vale Salles, 40, a car washer who used to earn about $20 a day cleaning walls of pichacao. He said his tormentors would spray their tags on buildings he had just scrubbed. He abhorred the job.

"As a cleaner of their mess," he said, "I felt horrible."