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.
Some 4.5m people use the New York subway every weekday
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 system has more than 468 subway stations.