The stepped-up security came as Secretary of State Hillary Rodham Clinton said that Al Qaeda was behind what officials had described as a possible car-bomb attack timed to coincide with the 10th anniversary of the Sept. 11 attacks. Reuters said that Mrs. Clinton, appearing at John Jay College of Criminal Justice in New York, had mentioned the "specific, credible but unconfirmed report that Al Qaeda again is seeking to harm Americans and in particular to target New York and Washington."

Vice President Joseph R. Biden Jr. had discussed the possible threat earlier in the day, on early-morning television news programs. "There are specifics — in that sense it was credible," he said on the ABC News program "Good Morning America," "but there's no certitude."

"There's no smoking gun," he said, "but we do have talk about using a car bomb."

Mayor Michael R. Bloomberg — who on Thursday night appeared with the city's police commissioner, Raymond W. Kelly, to announce that vehicle checkpoints would be set up in time for the morning rush — went to his office by subway on Friday. Of the latest threat, the mayor said, "It's serious, but I think the right answer is to go about your business."

"We've got to make sure we don't let the terrorists take away our rights without any terrorism," the mayor said on his weekly radio program on Friday morning. "If you do lock yourself in your house because you're scared, they're winning. If you don't let somebody else pray, or say what they want to say, or you deny any rights to certain people — that's exactly what they want. I don't think we should do that."

Still, as New Yorkers absorbed the news that officials were analyzing intelligence about the possible threat, the metropolitan region had more of the feel of being on edge than it had in recent months.

The increased police presence forced drivers heading toward Manhattan on the Brooklyn Bridge to squeeze through a single-lane gantlet as police officers walked between the cars, singling out some for a closer look. In Lower Manhattan, just a few blocks from ground zero, police vehicles with flashing lights stood in front of the American Stock Exchange.

"It's good," said Wolfgang Klebe, who runs a shipping business in Lower Manhattan, as he watched the officers on Friday morning. "They have to do this."

The report on the threat came after several quiet weeks in which officials said they were scanning intelligence with extra vigilance before the anniversary, but had found nothing credible. A spokesman for the Department of Homeland Security, Matt Chandler, said on Thursday that a notebook seized in the raid on Osama bin Laden's compound in May speculated about mounting an attack 10 years after 9/11 or on another symbolic date.

"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days," Mr. Chandler said. "Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots under way.

One law enforcement official said that the initial intelligence report described how at least three suspects, one of them an American citizen, had left Afghanistan and entered this country by air last month. Intelligence agencies have not confirmed the report or identified those involved, the official said.

But even before the new threat was disclosed late Thursday, Mr. Kelly had spoken of the Police Department's efforts to step up public safety and security around the Sept. 11 anniversary. In a speech on Wednesday, he outlined some of the initiatives: Creating a "frozen zone" from West Street to Broadway and from Murray Street to Albany Street in Lower Manhattan; posting bomb technicians in strategic areas; deploying divers in the water, near bridge stanchions and other sensitive sites; and putting "thousands of extra officers" on duty as extra eyes and ears.

Al Baker, Michael Barbaro, David W. Chen, Colin Moynihan and Scott Shane contributed reporting.