As New York Mayor Michael Bloomberg promised, security has been ramped up following the reveal of credible intelligence that al-Qaida militants in Pakistan may be pursuing a plot to carry out car or truck bombings in Washington and New York City to coincide with the 10th anniversary of the Sept. 11 attacks.

Police cruisers have taken up position on a midtown block of Lexington Avenue Friday morning, as officers stopped trucks and other vehicles for inspection, Bloomberg reported. And in Brooklyn, U.S. Marshals armed with machine guns guarded the federal courthouse; increased security was also spotted in from of the nearby city emergency management office.

We have already had a full complement of people working shifts because of the Sept. 11 anniversary prior to this, said Jim Margolin, a spokesman for the FBI's New York office. We are taking the logical investigative measures to assess this threat.

New York police have also beefed up security at bridges and tunnels, and are setting up vehicle checkpoints; drivers were required to open the storage space on their trucks when stopped at the Lexington Avenue blockade. Police are also implementing bomb sweeps of parking garages and towing more illegally parked cars.

New York commuters have also been told they will see a show of force at major transportation terminals, including Grand Central, Penn Station, and near the Port Authority. Police officers will extend their shifts by four hours at least through Sept. 12, Commission Ray Kelly said.

It has not been decided whether the extra security would be in place past Monday, according to supervisor deputy U.S. Marshal James Elcik. At the subway station in the Port Authority bus terminal at 42nd street, two police officers stood guard at the turnstiles while two other patrolled the platform. Police officers in a group of four stood on the platform at Broadway and Murray streets in lower Manhattan giving directions to tourists making their way to and from ground zero four blocks to the south, Bloomberg reported.

We have threats all the time, the mayor said during his weekly appearance on WOR radio. Each time we increase our security, which obviously we had done for this. Are we increasing a little more? Yes, we're increasing a little more, but there's a limit to how much you can have, just because you can't have a cop on every corner. But remember, a lot of the precautions we take, you don't see - undercover and camera and radiation detectors, using technology and undercover.

Bloomberg rode the subway as usual Friday morning, an effort to reassure the city's eight million residents that life should continue as normal.

We don't want al-Qaida or any other organization ... to take away the freedoms without firing a shot, Bloomberg told The Associated Press, getting off at the City Hall stop in lower Manhattan, near the Brooklyn Bridge. Bloomberg urged New Yorkers to just go back to work. And leave it to the professionals.

U.S. officials said they received the intelligence of a possible attack within the last 48 hours, and that they are taking it seriously because of the proximity to the 9/11 anniversary, The Wall Street Journal reported.

The alleged terror plot is believed to have been initiated by Ayman al-Zawahiri, the new al-Qaida chief, who pledged to avenge Osama bin Laden's death earlier this year. ABC News is reporting that at least one of the individuals is a U.S. citizen and one official said that two of the individuals may have had U.S. documentation -- whether green cards or passports was unclear.

A U.S. official, speaking on condition of anonymity, told Bloomberg that the intelligence concerns a possible vehicle-borne attack, perhaps on a transportation hub or bottleneck, but said the options could be broader than a car or truck bombing.

The threat was described as specific and credible by federal officials.

Al-Qaida has shown an interest in important dates and anniversaries. In this instance it is accurate that there is credible, specific but unconfirmed information, said Janice Fedarcyk, FBI Assistant Special Agent in Charge of the New York office, at a Thursday evening press conference.

As another indication of the severity of the threat, President Obama was briefed on the threat multiple times Thursday and directed U.S. intelligence officials to take all necessary steps to ensure vigilance, according to the White House.

On Friday, Vice President Joe Biden said the first active plot timed to coincide with the commemoration of the 10th anniversary of Sept. 11 is a real threat. Appearing on CBS' The Early Show, Biden said intelligence investigators are following every possible lead in the attempt to diffuse the threat.

People should be alert, they should not alter what they're doing, he told CBS, noting we have significant security, local police and federal agencies working on this.

In another interview, with Good Morning America, Biden said the revolved around New York City and Washington, and that a car bomb might be involved in the threat.

We do have talk about using a car bomb. We've been told that was an intention ... from a credible source, Biden said. But we do not have confirmation of that.

The threat comes as Americans and officials across the country, including President Obama and former President George W. Bush, are set to commemorate the Sunday anniversary with high-profile events at each site of the attacks -- in New York City, Shanksville, Pa., and the Pentagon in Virginia.

Though badly weakened after the May death of former al-Qaida leader Osama bin Laden, U.S. officials acknowledge that al-Qaida in Pakistan is still capable of operating with a handful. Current leader al-Zawahiri and top lieutenant Abu Yayha al-Libi are of particular concern.

But three other current al-Qaida leaders are also believed to present a threat to the U.S., as they have lived here in the past, the WSJ reported. They are Adnan el Shukrijumah, who is alleged to have been involved in the 2009 New York subway bomb plot; Jude Kenan Mohammad, an American alleged to have helped recruit five Alexandria, Va., men; and Adam Gadhan, an American who serves as an al-Qaida spokesman.

More intelligence about potential attacks is expected in the coming days, Matthew Chandler, a spokesman for the Department of Homeland Security, said in a statement.

Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots underway, Chandler said. Regardless, we take all threat reporting seriously, and we have taken, and will continue to take all steps necessary to mitigate any threats that arise.