Oct. 6 (Bloomberg) -- New York police stepped up counter- terrorism patrols on the city's subways after receiving a ``specific threat'' of a potential attack, Mayor Michael Bloomberg said today.

``This is the first time we have had a threat with this level of specificity,'' Bloomberg said at a news conference at police headquarters. Police Commissioner Raymond Kelly said that while the threat wasn't fully corroborated, he and FBI officials considered it credible enough to warrant increased precautions.

``It was more specific as to target'' than any threat before, Bloomberg said, without detailing the nature of the threat. ``It was more specific as to timing, and some of the sources had more information that would lead one to believe that it was not the kind of thing that appears in the intelligence community every day.''

Bloomberg said the information ``originated from overseas'' and came through the Federal Bureau of Investigation, but didn't give details. The city remains at the same stage of heightened alert that it has since Sept. 11, 2001, when terrorists attacked the World Trade Center, killing 2,749 people.

Police will increase bag searches instituted after the London subway bombings in July, and will be watching baby carriages, briefcases and other containers, Kelly said. Plainclothes and uniformed officers will ride the trains and be posted at stations, he said.

Searches

``You may rest assured that we will search your backpack with the same diligence that we would search anything else that you had with you that could conceivably conceal something,'' Bloomberg said. ``That person sitting next to you on the subway may be a police officer.''

Kelly asked the public to report anything suspicious and try to refrain from bringing bags and packages into the subway system. New York's subway system, the largest in the U.S., carries 4.5 million passengers on an average weekday, according to the Metropolitan Transportation Authority Web site.

New York police and FBI agents continue to investigate the threat, Kelly said. Bloomberg said authorities have no reason to believe that anyone intending an imminent attack is in New York now.

Mark Mershon, special agent in charge of the FBI's New York City field office, said ``classified operations have partially disrupted this threat.''

``There are ongoing investigative and enforcement activities and there's the possibility that this particular case will be resolved in the coming days,'' Mershon said.

`Doubtful Credibility'

Some officials played down the urgency of the threat. In Washington, Homeland Security Department spokesman Brian Doyle said ``there is doubtful credibility to this source.'' Senator Charles Schumer, a New York Democrat, characterized the threat as ``specific but not corroborated and not of the highest credibility.''

Kelly declined to say whether he would advise New Yorkers to avoid the subways, while Bloomberg said, ``Tonight, I'm going to take the subway uptown.''

``We live in a dangerous world,'' Bloomberg said. ``We've tightened our security before. Hopefully, what we'll find is that any plans that did exist have been dissolved by the actions that have been taken by various security groups around the world.''

The New York warning didn't prompt immediate reaction in some other transit agencies. California's Bay Area Rapid Transit, which serves the San Francisco region, hasn't issued any kind of security alert, said spokesman Jim Allison. Los Angeles transit officials also said there was no heightened alert, though they were consulting with the sheriff's office to discuss the situation, spokesman Rick Jagger said.

The mayor is founder and majority owner of Bloomberg News parent Bloomberg LP.

To contact the reporters on this story: William Goldschlag in New York at wgoldschlag@bloomberg.net ; Henry Goldman in New York at hgoldman@bloomberg.net