It was a graffiti artist's dream come true: 10,000 square feet of concrete and a permit to paint. Families brought their kids to watch as hundreds of muralists, using their own materials and working for free, sprayed technicolor shades on the steep banks of an ugly, manmade riverbed.

Not everyone was pleased, however, with the results of the civic-minded effort, which had the city's blessing but has rekindled debates over whether Los Angeles County should condone a practice it pays millions to combat.

Some politicians protested that parts of the mural are obscene and have attracted gang-related tags in a city where graffiti already mars homes, sidewalks and buildings. The county has given organizers until Wednesday to whitewash the mural, and neither side is backing down.

"It would be beautiful if the river went back to its natural state and was actually a river and a park," said Alex Poli, a graffiti artist and gallery owner known as "Man One." "But right now we have concrete walls, so the next best thing is to beautify it with art."

The site in question, a concrete canyon where a tributary, Arroyo Seco, meets the Los Angeles River, is surrounded by an industrial neighborhood on the edge of downtown and, like most of the river's 51 miles, is hemmed in by artificial banks to control floods.

To obtain the permit from a maze of local governments and regulatory agencies, Poli enlisted the Friends of the Los Angeles River, an environmental organization that works with the multiple agencies that control the river.

Poli organized the public art project on a sunny weekend in September, and the artists created a canvas full of bold, abstract graffiti script and some edgy imagery: a sorcerer in a hoodie sweatshirt conjuring a spray can, an angel cradling a man, a pig in a suit smoking marijuana, the Hollywood sign in flames and scantily clad women.

County Supervisor Gloria Molina promptly demanded the mural's removal, complaining that some of the images were inappropriate for a public art display near where city planners want to build bike paths. The environmental group's mission is to protect the river, and "this seemed like an odd way to do it," said Roxane Marquez, a Molina spokeswoman.

Marquez said Poli hasn't kept his promise to organize a volunteer touchup crew to keep the surrounding concrete pristine and free of gang tags and extra graffiti.

Poli said the politicians don't understand the difference between graffiti and graffiti art, which is exhibited in museums and galleries around the world.

"People still have trouble considering it art because we use a spray can," he said.

In mid-October, some of the murals were whitewashed without warning. Molina and the Department of Public Works denied involvement, but in December, Molina got the county Board of Supervisors to pass an emergency motion giving the Friends of the Los Angeles River 90 days to paint over the murals or pay up to $70,000 for their removal.

County crews removed about 60 million square feet of graffiti in 2006 at a cost of about $32 million, county officials have said.

The Friends group stands by the idea of having art by the river, spokeswoman Shelly Backlar said. But the organization, which is scrambling to rebuild its stock with the county and the agencies that supervise the river, concedes some of what the artist put into the mural might not belong there.

"It's their permit and their event, and we've been pulled in because of the work that we do," Backlar said. "It's not what we thought it would be."

City Councilman Ed Reyes, who originally supported Poli's project and authorized the permit, said he regrets that decision because he believes the art has attracted gang members, who have added their tags to the riverbed walls.

The graffiti "spilled out of the river channel, into the sidewalks, onto the handrails, into buildings," Reyes said. "Before it was a neutral place, but now we have clear indicators that rival gangs and taggers are showing up there."

More tagging has steadily accumulated at the Arroyo Seco site since last fall. Other artists have primed their own pieces of concrete and added to the project, extending the murals a few dozen yards.

Poli condemns taggers but sees the more ambitious work as copycats - students learning from the masters. Tagging increased after parts of the mural were whitewashed, including offensive images directed at Molina and county officials.

"The county needs to wake up," said Kalen Ockerman, who paints under the name "Mear One." "The rest of the world is busy paying kids to do this stuff," on album covers and billboards.

Poli considered painting over the murals, "because of all the grief." He's also talking to lawyers, hoping that a strongly worded letter will stop the county from billing the environmental group or his gallery.

"We did nothing illegal and we had permits," he said. "We're in the business of creating art, not destroying it."