Wall Street is keeping its eye on stocks - and bombs.

Police brass began posting uniformed officers at the New York and American stock exchanges around the clock in response to recent terrorist threats, law enforcement sources said Tuesday.A small army of plainclothes officers also is encamped on Wall Street, added the sources, who spoke on the condition they not be identified.

Mayor Rudolph Giuliani said he spoke with the director of the FBI and that "every single precaution is being taken."

"From the point of view of law enforcement," he added, "the less said about it, the better."

The sources refused to say who made the threats on Wall Street or if they stemmed from the trial of Sheik Omar Abdel-Rahman and his followers, accused of planning a series of bomb attacks in New York.

But Wall Street workers said security had visibly increased beginning Feb. 2, the same time one of the defendants was negotiating a plea bargain. Siddig Ibrahim Siddig Ali said in court that he wanted to tell prosecutors about "a few things that are happening out there that I don't want to be part of."

Siddig Ali's cryptic warning coincided with a New York Police Department memo to private security forces urging a "heightened sense of awareness" at the approach of the second anniversary of the World Trade Center bombing of Feb. 26, 1993.

As a result, "The financial district is probably the most secure part of the city right now," said former Transit Police Chief Michael O'Connor, now head of security for a downtown business improvement district.

On Tuesday, police stopped delivery trucks to check papers before letting them through to the back of the New York Stock Exchange, not a usual practice.

Barricades had been placed to prevent vehicles from parking in front of the exchange.

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ADDITIONAL INFORMATION

Matchbook match?

The latest tool in the war against terrorism: matchbooks.

In an effort to find a prime suspect in the World Trade Center bombing, U.S. counter-terrorism experts spread 37,000 matchbooks with his photo throughout the Middle East and Pakistan.

Some suggest that the pocket-size wanted posters helped lead to last week's capture of Ramzi Ahmed Yousef in Pakistan.

"We basically blanketed the country," said Robert Born, a State Department terrorism specialist. "Smoking is on the decline in the U.S., but that's not the case overseas."

Yousef, 27, had been a fugitive for the two years since the trade center blast.