NEW YORK — City officials Thursday increased security in New York's subways, citing what they called a credible terrorist threat against the nation's largest mass transit system.

But authorities with the Department of Homeland Security in Washington downplayed the seriousness of the threat, saying it was of doubtful credibility.

"We have never before had such a specific threat to our subway system," Mayor Michael R. Bloomberg said during an afternoon news conference.

New York officials said the police presence would be increased at many of the city's 468 stations. For the first time, police urged New York commuters to refrain if possible from bringing into the stations backpacks, baby strollers, briefcases or bags that might be used to hide explosives.

Bloomberg, speaking at police headquarters with NYPD and FBI officials, said the importance of the threat was "above the normal level" because of the detail involved.

Officials were reluctant to elaborate, however, saying that information about the potential threat had grown out of a "classified operation" overseas. Bloomberg said no suspects had been arrested in New York or anywhere else in the U.S.

New York officials knew about the threat for several days but did not make any statements that could have "jeopardized the lives" of agents investigating the plot, Bloomberg said. He added that one news organization had learned of the threat two days ago but did not make the news public at the request of government officials.

"Classified operations have partially disrupted this threat," Mark Mershon, assistant director of the FBI's New York office, said at the news conference. "The FBI and other U.S. personnel continue to work around the clock," he said, adding that the threat might be "resolved in the coming days," without offering further details.

Homeland Security officials "received information regarding a specific but noncredible threat to the New York City subway system," said spokesman Russ Knocke.

"The intelligence community has concluded this intelligence is of doubtful credibility," he said.

Knocke added that federal officials had shared information about the threat with New York officials "out of an abundance of caution" and said his agency recognized that "New York City operates at a very high level of security out of what they deem appropriate for their city."

But he stressed that "there are no plans to alter the national threat level or the threat level for New York City at this time."

(The national color-coded threat level remains at yellow; New York is at the higher level orange, as it has been since the Sept. 11 terror attacks.)

After an evening call between New York authorities and FBI Director Robert S. Mueller III, the FBI said it was working closely with local, state and federal authorities to further explore the potential threat.

FBI spokesman Richard Kolko had no comment, however, on the differing NYPD and Homeland Security assessments, saying only that his agency "is aware of the information" and that it "has not been corroborated or discounted."

Federal officials have clashed with their local and state counterparts on threat assessments in the past. In November 2001, then-California Gov. Gray Davis announced that there was a credible threat to one or more of California's suspension bridges, angering FBI officials who told law enforcement agencies that there was no credibility to the threat.

The conflicting views of the New York subway threat underscored the differences between Homeland Security analysts and the NYPD, which has beefed up its own anti-terrorism intelligence division in the wake of the Sept. 11 attacks. Although the department cooperates with federal officials, it prides itself on independence.

Police Commissioner Raymond Kelly, for example, has sent NYPD intelligence officers to the scenes of terrorist attacks in the London and Madrid subways. He has a separate squad of experts monitoring international incidents and their potential links to New York.

Kelly said the threat disclosed Thursday was unlike any that had been received before, and thus required officials to alert the general public.

"We have received information" suggesting that "the New York subway system may be the target of an attack in the coming days," Kelly said. "We ask the public to report anything suspicious, and we will continue to step up security until further notice."

The department's current program of randomly stopping commuters and inspecting packages will be expanded, Kelly said; there will be increased numbers of police officers, both in uniform and undercover, riding trains throughout the city.

New York officials beefed up transit security after the London subway bombings in July.

But they conceded that it would be impossible to fully protect the system, which is used by 4.5 million people each day.

New York could not "realistically" expect to install metal detectors in all stations, Bloomberg said.

"We have done and will continue to do everything we can to protect this city," the mayor said, noting that he will continue to use the subway.

He planned to take the subway to an evening event and would ride the subway to work today, Bloomberg told reporters. "We will spare no resource; we will spare no expense."

Annotazioni − Times staff writers Nicole Gaouette and Josh Meyer in Washington contributed to this report.