After the federal government warned of a potential Al Qaeda attack, as the city shifted into Thanksgiving holiday mode, the New York Police Department (NYPD) and the Metropolitan Transportation Authority have stepped up patrols in the city's subways and trains.

The increased police presence results from a "plausible", but not specific, report that Al Qaeda terrorists had discussed the use of suicide bombers against city mass transit operations. Police sources said the information came from a Pakistani citizen arrested overseas within the past week.

According to NYPD spokesman, Paul Browne, "The New York City Police Department is aware of an unsubstantiated report indicating that Al Qaeda terrorists discussed targeting mass transit in New York City and vicinity. We have no information indicating that these discussions advanced beyond the aspiration stage."

The horrors of terror bombings in recent years in London and Madrid have accentuated the exposure of mass transit systems to attack.

The vulnerability of the city's tightly packed passenger trains and subway cars has long been a source of concern for cops, and target for would-be terrorists - the city has more than 450 subway stations that handle millions of commuters every day.

A person briefed about the proceedings, speaking on condition of anonymity due to the sensitivity of the intelligence-gathering work, said the threat may also be directed at the passenger rail lines running through New York, like Amtrak and the Long Island Rail Road. The threat surfaced on one of the busiest travel days of the year and when tens of thousands of tourists are in New York City for the Macy's Thanksgiving Day Parade.