A federal judge is taking a skeptical view of the city’s latest anti-graffiti effort, giving generous consideration to First Amendment concerns over a law forbidding 18- to 20-year-olds from carrying spray paint or large markers in public areas.

Judge George Daniels of U.S. District Court in Manhattan promised to decide whether to suspend the law on Monday, when the city is scheduled to submit information from the Police Department about its enforcement efforts and the effects of the law.

At the end of yesterday’s hearing, Judge Daniels gave marching orders to the city lawyer who is arguing the case, Virginia Waters: “I suggest you leave here this afternoon, walk on over to the Police Department, and tell the commissioner how important this is.”

When Mayor Bloomberg approved a package of anti-graffiti measures last December, including the law being challenged, he described them as common sense. But the law, which carries a penalty of up to a year in jail, was challenged Tuesday in a lawsuit backed by fashion designer Marc Ecko. The suit claims that the law hampers young artists who use spray paint and large markers in their art work.

Judge Daniels’ line of questioning suggests he is troubled by the inconveniences the law places on young art students in New York City.

How, he wanted to know, is an art student to travel to school with supplies like a can of spray paint?

“An art student does artwork at home or has an art school buy supplies,” Ms. Waters answered. “My son had an art project he had to use spray paint for. I went out and bought it for him.”

The city’s answer did not impress Judge Daniels, who said at one point: “It seems troubling we can invite a 20-year-old to vote for president of the United States but not possess a magic marker.”

At another point, he pictured himself in the shoes of young graffiti artists, such as those represented in the lawsuit: “I’m standing on the subway platform. I’m a 20-year-old going to class,” he said. “You’re saying that any police officer can arrest me? … The only way I can defend myself against a year in jail is to put on a defense in a trial?”

Mr. Bloomberg has demonstrated as much anti-graffiti zeal as Mayor Giuliani before him, and last year created an 80-member task force to address the problem of graffiti scrawl, which has long plagued New York’s public places and subway cars.

Ms. Waters argued that there was no fundamental right to possess a marker or can of spray paint. “Legislative enactments, if they’re rational, have to be given deference by the court,” she said.

Neither Ms. Waters nor the attorney for the plaintiffs said they were aware of any arrests made under the new law, which went into effect January 1. The lawyer for the plaintiffs, Daniel Perez, appeared in court wearing a tie bearing the image of a can of spray paint. He said one of the plaintiffs, Lindsey Vincenty, had made it.