New York ? A terrorist plot to attack the subways with bomb-laden baby carriages and briefcases – the most specific threat ever made against the city – triggered a massive security crackdown Thursday.

Details of the chilling scheme came from an informant, but officials kept it secret for three days while suspects were rounded up in Iraq and security was quietly ramped up here, sources said.

One source said the group was aiming to strike in mid-October. Another official said the target date was this holiday weekend, and that the plotters were linked to al-Qaida.

Though Homeland Security officials in Washington downplayed the threat, deeming it of “doubtful credibility,” FBI and New York police officials said the level of detail gave it more significance than the usual terrorist chatter.

“It was more specific as to target, it was more specific as to timing,” Mayor Michael Bloomberg said as police began visibly beefing up police presence in the subways. “It was not the kind of thing that appears in the intelligence community every day.”

New York police search a commuter's suitcase at the Union Square subway station Thursday in New York. City officials stepped up mass transit security, saying they had received a credible threat that New York's subway system could be the target of a terrorist attack in coming days.

FBI Assistant Director Mark Mershon said the report his agents got was “so on point” that he was compelled to warn city police.

A tipster who has given accurate information to the FBI in the past exposed the plot and named three operatives, a law enforcement source said.

Up to a dozen terrorists planned to come to the city and assemble into cells of suicide bombers who would attack the subways – hiding explosives in strollers and briefcases – a senior intelligence official said.

The terrorists envisioned a series of attacks that would be “London-scale, not World Trade Center-scale,” the intelligence official said. “They’d be terror strikes instead of spectaculars.”

Federal authorities rushed to derail the plot, and two of the three operatives were arrested south of Baghdad Thursday – freeing the police to tell the public about the threat, sources said.

Police will sweep train cars and search passengers, with particular attention to briefcases, baby strollers, luggage and containers that could conceal bombs.