People in Philadelphia caught with small amounts of marijuana or smoking it in public may soon face no more than a written citation and a modest fine as the City Council moved Thursday to decriminalize the drug after a reversal by Mayor Michael A. Nutter.

The mayor, who had scoffed at arguments that black men are unfairly targeted in arrests involving marijuana, announced his support of decriminalization this week. He had declined to sign a bill the Council passed in June, saying it needed more study.

Under the measure, which if approved would be effective Oct. 20, people stopped with less than 30 grams of marijuana, about one ounce, will be fined $25, and those caught using it in public will be fined $100 or be required to perform up to nine hours of community service.

In neither situation will violators be arrested, taken to police stations, fingerprinted or left with a criminal record — all of which stigmatize small-time users, making it harder to find and keep jobs or attend college, advocates of decriminalization say.