The fashion designer Marc Ecko, a one-time graffiti artist, is painting an ugly picture of a new law that Mayor Bloomberg enthusiastically backs.

Mr. Ecko is threatening to sue the mayor and Council Member Peter Vallone Jr. for imposing legislation that makes it illegal for anyone under 21 to carry aerosol spray paint, etching tools, broad tip permanent markers, and other graffiti tools on public property.

The law, which went into effect in January, is part of an overarching city crackdown on graffiti that was initiated when Mayor Giuliani began his quality of life push.

Mr. Bloomberg has taken the war on graffiti to the next level. Last year, he created an 80-member task force to tackle graffiti and in 2003 his administration painted over 16 million square feet of scrawl on subways, buildings, and metal store shutters.

A spokesman for Mr. Ecko, who won a First Amendment lawsuit against the city over the summer after the mayor attempted to shut down a graffiti themed block party, declined to discuss his plans yesterday. He has a press conference scheduled for today.

An e-mail he sent out said the city’s graffiti regulation “impacts the First Amendment rights of aspiring artists under the age of 21.” The message said seven artists who are under 21 will ask a federal judge to prevent the city from enforcing its new anti-graffiti measures.

A prominent First Amendment lawyer, Floyd Abrams, said a law that makes it a crime for those who are under 21 to carry spray paint and other tools sounds “constitutionally dubious.”

“I think it’s a significant stretch from a constitutional perspective that you can be deemed to have committed a crime by just carrying spray paint and etching tools on public property,” Mr. Abrams said.

He said, however, that a sweeping First Amendment case would probably not succeed and that the artists might have more success arguing that the city government is overstepping its authority.

Mr. Vallone said he has full confidence that the law will stand up against a legal challenge. He accused Mr. Ecko of trying to drum up publicity for his clothing line and products.

“He’s not filing the suit to protect any student doing an art project,” Mr. Vallone said yesterday. “The bottom line here is profits, not the First Amendment, it’s profits.”

“I admit that this is a law that pushes the envelope,” Mr. Vallone added. “But that’s what we need here in New York City. A similar law to mine has been upheld in Chicago, so I have confidence that the courts will uphold it here.”

The chief of the city’s administrative law division, Gabriel Taussig, defended the law, but said the city would evaluate the legal papers once they are received.

“The law strikes a proper constitutional balance between First Amendment rights and the need to control the longstanding plague of graffiti,” he said in a statement.

Over the weekend, Mr. Ecko was the subject of other press reports for producing a two-minute Internet video that depicts a graffiti artist spray-painting the words “Still Free” on what appears to be Air Force One. The video looks so real that officials at the Air Force had to investigate it before dismissing it as a prank, press reports said.