Security is being stepped up on New York's underground railway network as city officials said they have received a "credible security threat".
Police chief Ray Kelly said patrols and searches were being stepped up, and he appealed to travellers to avoid using bags, briefcases and pushchairs.
Mayor Michael Bloomberg said the threat was the most specific yet, and was related to the next few days.
He urged people to remain vigilant and to report anything suspicious.
However, in Washington, Homeland Security Department spokesman Russ Knocke appeared to contradict the New York officials when he told the Associated Press news agency that "the intelligence community has concluded this information to be of doubtful credibility".
"We have never had before a specific threat to our subway system," Mr Bloomberg said, adding that he still felt secure enough to take the subway home after work and would take it back again in the morning.
"Its importance was enhanced above the normal level by the detail that was available to us from intelligence sources, he added.
Officials said details of the threat were classified, however Mr Bloomberg said it came from overseas.
"I want to assure New Yorkers that we have done and will continue to do everything we can to secure this city," Mr Bloomberg added.
Mr Kelly said that the terror plan had been "partially disrupted" by the additional measures being put in place, and that the authorities were working around the clock to "resolve this threat".
An existing policy of random searches on the subway would be intensified, he said, with a particular emphasis on baggage and pushchairs.
He added that the city's threat level would remain at "orange" - the second highest level of alert and the level it has been since the attacks on the city's World Trade Center on 11 September 2001.
An estimated 4.5m people use the New York subway every weekday.
The NYPD first learned of the potential threat to the city's subway system on Monday.
The details, as is often the case, were somewhat sketchy, but there was enough specificity in the briefing from the FBI to believe the threat was real.
According to city sources -- who gave an account somewhat different from federal officials -- several government agencies then went into action to stop the plot.
At the time, however, the decision was made not to go public with the news. According to city sources with knowledge of the case, an informant who provided details about the plot said the attack would occur late in the week, giving authorities enough time to potentially catch the suspects before they could be tipped off.
Quickly, the FBI, CIA and U.S. military put together a joint operation that resulted in the arrest in Iraq Thursday of at least two of the three operatives whose names had been provided by the informant, according to a city official with knowledge of the case.
The arrests prompted a massive show of force on behalf of the NYPD, with cops from a number of specialized units gathered en masse at several locations before being sent out into the nation's largest subway system.
A federal source, however, said officials learned of the threat following a U.S. military-CIA raid on an al-Ansar safe house in or near Bagdad earlier this week.
"They captured some of the guys and one of them told them about the alleged plot," said the source, who works in counterterrorism and was brief by intelligence officials Thursday.
As many as 19 people may be involved in the plot, according to the counterterrorism source, and while there was no immediate confirmation that any of them are on U.S. soil, other sources said the NYPD and FBI have not ruled out that possibility.
The reliability of the source of authorities' information, meanwhile, was up for debate Thursday.
The city official described the informant as "someone who has provided accurate information before," while the counterterrorism source said terrorism analysts are uncertain about the veracity of the information.
Indeed, a Homeland Security Department spokesman said "the intelligence community has concluded the information to be of doubtful credibility."
And Mark Mershon, head of the FBI's New York office, described the informant as someone who "truly believes this to be factual."