New York police have amassed a display of force in response to intelligence about a car-bomb plot linked to the 10th anniversary of the September 11 attacks.
Officers armed with automatic weapons were stationed on Friday at city landmarks including Wall Street, Times Square and the 9/11 memorial site where the Twin Towers once stood.
Checkpoints snarled traffic in much of the city, Al Jazeera's Asad Hashim reported from the city's financial district.
"The NYPD has set up checkpoints on the roads in many locations, and there are also random inspections taking place in the subway system, with bag checks set up at major stations," he said.
US officials have called the threat "credible but unconfirmed" and timed to the anniversary of the hijacked plane attacks that killed 2,995 people in New York City, Washington and Pennsylvania.
There was reason to believe threat may be linked to al-Qaeda leader Ayman al-Zawahiri, a government source told the Reuters news agency on Friday, speaking on condition of anonymity.
Al-Zawahiri took the reins of al-Qaeda after Osama bin Laden was killed in May in a US raid in Pakistan.
A manhunt was under way for two or three suspects, officials said, adding that there has been no changes to President Obama's plans Sunday to visit New York, Shanksville in Pennsylvania and the Pentagon to commemorate 9/11.
New Yorkers who have grown accustomed to bag searches at subway stations and random displays of police presence encountered increased vigilance after the threat, which prompted President Barack Obama to order a redoubling of US counterterrorism efforts.
Near the site known as Ground Zero, where a new World Trade Center is under reconstruction, police established a checkpoint behind the historic Trinity Church, stopping vehicles, opening the cargo bays of lorries and checking drivers' licences.
Similar checkpoints went up at Times Square, Columbus Circle, on the southern edge of Central Park and outside the Macy's department store in midtown, creating traffic jams all over Manhattan.
"I think for our safety it is good," Eva Kurzawska, 57, said as she watched irritated drivers a checkpoint in the midtown area.
"The commute on the train this morning was horrible but it was worth it because we are being protected," Mario Vigorigo, 42, a wireless manager from the borough of Brooklyn, said.
Sam Ginzburg, a senior trader at First New York Securities, said warnings of a potential attack was one factor unnerving traders before the weekend.
"There is an extreme amount of negativity," he said as US stocks fell on Friday.
A senior law-enforcement official said police patrols and security will be stepped up beginning at 3pm EDT [1900 GMT] to coincide with the evening rush hour.
The operation will involve a "big show of force" which will include teams of officers armed with heavy weapons.
While the rush-hour operation had been planned some time ago, the forces and tactics deployed were increased after authorities received the intelligence threat this week.
'Terrorism is theatre'
The intelligence included possible threats of attacks targeting subways or commuter trains or possible car bomb attacks in New York or Washington.
"We have to be concerned. Terrorism is theatre and this is a stage, right now probably the world's biggest stage," Raymond Kelly, New York City police commissioner, told CNN.
"We have the opening of the 9/11 memorial, the president and two former presidents here, obviously a lot of high profile public officials will be here, so we have to be concerned."
In addition to the vehicle checkpoints, police would assign additional officers to cover bridges and tunnels, deploy radiation detectors and employ bomb-sniffing dogs, Kelly told NY1 television.
Joseph Biden, the US vice-president, said on ABC's Good Morning America programme on Friday morning: "We don't have the smoking gun but we do have talk about using a car bomb".
A counterterrorism official said recent threat information came from Pakistan's tribal areas.
Documents discovered in Osama bin Laden's compound in Abbottabad, Pakistan, after he was killed in a raid in May by Navy SEALs, highlighted his persistent interest in attacking the US around the anniversary of the 2001 attacks.
But it is unclear if the plans ever evolved beyond aspiration.
Use of force
America will not shy away from using military force if necessary against terrorist groups such as al-Qaeda, Hillary Rodham Clinton, the US secretary of state, said on Friday in a speech at John Jay College of Criminal Justice in New York.
"Ultimately, this rests on the shoulders of the American people themselves," she said.
"If we can do those things, if we can be that country, then as much as all the daring night raids, successful prosecutions, persistent diplomacy, and targeted development, that will keep us safe, keep us strong, and keep us great."
Clinton also said that the Arab Spring revolutions against regional dictators showed that Arabs themselves were rejecting al-Qaeda's call for violent upheaval.
She said that the remaining threat required a sophisticated approach, "what I call a 'smart power' strategy that integrates all our foreign policy tools".