NYPD officers are continuing to inspect vehicles citywide as law enforcement officials investigate a reported al-Qaida car bomb plot aimed at bridges or tunnels in New York or Washington on the occasion of the 9/11 tenth anniversary.

The checkpoints caused massive traffic tie-ups at points throughout the city Friday, leading to some frustration among motorists.

One truck driver on the West Side Highway told NY1 he had been stopped five times Friday. "I think it sucks. It holds up all the traffic," he said.

However many were forgiving of the extra security.

"It's necessary because we need to be secure in this country," said one motorist.

Officials have not confirmed any details of the threat, but a senior U.S. official told the Associated Press Friday night that the plot was thought to involve three men, two of whom may be U.S. citizens of Arab descent or may be traveling with U.S. papers.

The source said the information, which has not been confirmed by investigators, suggested that the men were intending to set off a car bomb or find another way of causing violence in New York City or Washington, D.C. It was possible that the plot was being developed with the help of al-Qaida leader Ayman al-Zawahri.

Mayor Michael Bloomberg has stressed that even though the threat is credible, it has not been corroborated and that New Yorkers should be vigilant but not change their everyday routines.

Homeland Security Secretary Janet Napolitano and Governor Andrew Cuomo also released statements asking New Yorkers to remain vigilant through the weekend.

Cuomo also said he is deploying additional state police to infrastructure sites as a precaution and coordinating on security efforts with MTA Chairman Jay Walder and Port Authority Executive Director Chris Ward.

"We're doing the preparation, we're doing more preparation, but we should not allow them to tarnish the spirit of what this anniversary celebration is all about, because that would be an infringement on the freedom that we seek to celebrate on Sunday," said Cuomo.

This comes as President Barack Obama announced today that he has extended for a year the national emergency declared on September 14, 2001, due to the continuing terrorist threat.

Security has increased at the city's major transit hubs, bridges, tunnels, landmarks and houses of worship, although much of those precautions were being taken anyway ahead of Sunday's anniversary.

The mayor said September 11th anniversary ceremonies will be held as scheduled and President Barack Obama is still visiting on Sunday the World Trade Center site, the Pentagon and Shanksville, Penn.

Police Commissioner Ray Kelly said he has increased surveillance of bridges and tunnels and set up vehicle checkpoints and doing bomb sweeps of parking garages.

"We’re doing a lot of things, most can be seen, but some quite frankly can’t be seen, to put in additional protection to keep the city safe," said Kelly.

Counterterrorism, transit and highway officers will work 12-hour shifts for several days, bag inspections are increasing in subway stations and the police are using more patrol vehicles with plate readers, bomb-sniffing dogs and radiation-detecting equipment.

Police are also towing illegally parked cars.

From the George Washington Bridge and along the West Side Highway down to Canal Street, security checkpoints have resulted in backed-up traffic, as police inspect around and underneath vehicles.

Columbus Avenue and other avenues on Manhattan's West Side have been shut down to one lane of traffic to make checkpoints possible and trucks were being searched in Times Square and Herald Square.

At Pennsylvania Station, there is no dramatic increase in security, but Port Authority police, the NYPD, the National Guard and other law enforcement agencies are on alert.

"Six or seven Amtrak police with very heavy-duty armor and guns," said an Amtrak passenger from Albany. "Got off the train and there were police everywhere, along with a dog as well."

The World Trade Center site also has extra NYPD and Port Authority officers and one of the NYPD's so-called "Eye In The Sky" units parked nearby.

Bloomberg said the city's increased security measures will not always be noticeable to laypersons.

Homeland Security officials said that information gathered during the raid on Osama bin Laden's compound in Pakistan showed al-Qaida was looking into attack the United States again, and was considering an attack on the anniversary of September 11th.