New York City’s transit system could be the target of a terrorist attack “in the next couple of days,” the police commissioner, Raymond Kelly, said at a press briefing yesterday with Mayor Bloomberg and an FBI official.

“This is the first time that we have had a threat with this level of specificity,” Mr. Bloomberg said. He also noted that this is the first time the city’s subway system is the intended target.

The officials were vague about the logistics of the proposed attack, saying the information is classified, but sources confirmed to The New York Sun reports that 19 operatives are planning to conceal explosives in briefcases in the subways. ABC News reported that one of three recently arrested Iraqi insurgents divulged the information.

Intelligence sources told the Sun that the information was acquired in Iraq and elsewhere and that the documentation came from the capture of electronics.

Mr. Bloomberg said there have been no arrests made in Manhattan in connection with the plot and that investigators “do not have any reason to believe” the insurgents are currently in New York.

The threat bears no connection to any religious holidays, Mr. Bloomberg said.

Rosh Hashana was celebrated Tuesday and Ramadan, the month long Muslim fast, started Wednesday; next Thursday is Yom Kippur, the Jewish people’s Day of Atonement.

While the mayor and other law enforcement officials discussed how they are responding to the threat, a spokesman for the Department of Homeland Security said he doubted its validity.

To address the threat, law enforcement officials are deploying additional uniformed and plain-clothes officers in subways, buses, and ferries. Police officers will board trains more often and random bag searches will be more frequent, with particular focus on briefcases, baby carriages, and luggage.

The authorities said the additional security measures will continue until further notice.

The police department will coordinate enforcement efforts with the Port Authority of New York and New Jersey, Amtrak, New York City Transit, and Metro-North Railroad.

Governor Pataki described his efforts to provide additional security to the city.

“We will be calling up hundreds of National Guard members, and both state troopers and MTA police will increase their presence on mass transit and there will be increased frequency of security screening,” Mr. Pataki said in a statement. He added, “I have issued an executive order that allows law enforcement officers from both Connecticut and New Jersey to be on-board incoming and outgoing New York commuter trains. These law enforcement personnel both uniformed and not, will provide an added measure of security for New York commuters.”

A spokesman for the Metropolitan Transportation Authority, Thomas Kelly, responded to the subway system threat by saying the authority’s police force stepped up manned and canine patrols on subway and commuter rail lines yesterday in cooperation with the Police Department. Areas of special concern were Grand Central Terminal and Penn Station, the nexus points for both subway and suburban rail lines, Mr. Kelly said. The transportation authority did not suspend service yesterday. “It’s rush hour as usual,” Mr. Kelly said.

Council Member John Liu, chairman of the council’s Transportation Committee, weighed in on the issue. He pressed for immediate public notification of legitimate threats to the transit system and said, in a statement, “This threat serves as another reminder for the MTA to speed up the installation and upgrade of [public-address] systems in our subways as well as other communication technologies to better inform the public in the event of an emergency.”

The authorities divulged the terrorism warning two days after the FBI received the threat to the subway system. They waited to release the information, Mr. Bloomberg said, because plots against the transit system were not expected to materialize over the last couple of days and they did not want to jeopardize the investigation.

Although the city has received threats of acts of terror in the past, this one was deemed legitimate because of its level of detail.

The long-shot Conservative Party mayoral candidate, Thomas Ognibene, said at a debate last night with Democrat Fernando Ferrer at the Apollo Theater that if there is a credible threat there should be an announcement. But, he said, it was “kind of sad that it had to happen on a night when we’re debating here, but these are things that are unavoidable.”

The city’s security level remains at orange, signifying a high risk of terrorist attack, where it has been since the 2001 terrorist attacks.

Law enforcement officials said they have made some headway in the current investigation. The assistant director in charge of the FBI’s New York office, Mark Mershon, said at yesterday’s press conference that the agency has been able to partially foil the impending threat.

“Classified operations have in fact partially disrupted this threat,” Mr. Mershon said.

Senator Clinton said in a statement: “It is imperative that New Yorkers know that our transportation infrastructure continues to operate with the highest security operations in place.”

The mayor assured that despite the news, he would be riding the subway. “Tonight I’m going to take the subway going uptown,” he said. “And tomorrow morning I’m going to do what I always do and get on the train and go to work.” When it was suggested that because of his security detail the mayor might be in a different position than the average straphanger, he responded that a security detail “would not help me if there was a terrorist attack.”