Teams of heavily armed police and counterterrorism squads will bar trucks from certain bridges and tunnels, establish checkpoints along Manhattan streets and double security around key office buildings, from the New York Stock Exchange to Citigroup Center and the United Nations.

In Newark, police in tactical gear and carrying assault rifles set up posts around the gleaming 24-story Prudential Financial headquarters, which is said to be a terror target. Police erected metal barricades closing streets on either side of the office tower and barring access to the sidewalk and curb in front.

The most stringent security measures will go into effect Monday morning, as officials warned commuters to expect delays on the roads and at garages and office security stations.

New Yorkers have lived in a state of orange alert since the terrorist attacks of Sept. 11, 2001. But this alert is different, confined as it is to five buildings in New York, New Jersey and Washington, after evidence emerged that terrorists apparently conducted extensive surveillance of these buildings.

"You might think of this as the enemy's intelligence report," Police Commissioner Raymond Kelly told a City Hall news conference. "There's no question that they've done a bit of research and they're serious."

City officials will bar trucks from the Williamsburg Bridge spanning the East River between Brooklyn and Manhattan by diverting the trucks to the Manhattan Bridge, where they will put in place extra squads of police. Likewise, Port Authority officials have barred trucks from the Holland Tunnel, which connects New Jersey and Manhattan.

The extra security may be harder to notice in neighborhoods such as Wall Street. On most days, the New York Stock Exchange already resembles an armed camp, with black cement barriers, bomb-sniffing dogs and police officers armed with rifles. But city officials will dispatch still more officers with their Operation Atlas counterterrorism teams to patrol streets and subway stations in Lower Manhattan.

The Coast Guard has dispatched several vehicles to patrol near the targeted buildings.

Elsewhere, 76 surge teams of police officers will continue to swarm certain blocks with squad cars and tactical trucks, sirens blaring and lights flashing. The idea, counterterrorism officials say, is to create a sense of uncertainty and disorient potential attackers.

"We will spare no expense and take no chances," Mayor Michael Bloomberg said. "Some of it will be visible and intentionally so. And some of it is virtually invisible."

This last was a reference to the fact that security officials have placed detectors throughout the city's business district in hopes of ferreting out possible chemical and radiological weapons.

Bloomberg and Kelly acknowledged that they face a clever and competent enemy and that the risk is that police will focus on securing a half-dozen buildings while terrorists ready an attack elsewhere. "There's no question there's a limited amount of resources," Bloomberg said.

Police officials urged landlords to secure their air-conditioning and ventilation systems against the possibility of a chemical attack. Still, Kelly said that intelligence reports lean more toward the possibility of a bomb attack. "Explosives are considered more likely at this time," Kelly said.

City and state officials mixed talk of imminent terror attacks with admonishments that New York and New Jersey residents should go about their lives and shop and eat and enjoy life in the city. That extends even to the buildings that may be targeted. At the 59-floor Citigroup Center on 53rd Street and Lexington Avenue, security officials closed the street-level glass atrium and 16 police officers surrounded the building. But the subway station beneath the building remained open, and a restaurant and theater were packed in a plaza at the foot of the building.

The seeming dissonance in the official messages has worn on many residents, quite a few of whom have come to discern a political agenda in such alerts.

Richard Murdock rode the F-line subway into Brooklyn on Sunday, studiously unconcerned about the hubbub around him. "I wonder if it has anything to do with this," he said, pointing to a newspaper headline that President Bush was readying new political ads attacking Democratic nominee John F. Kerry. "I believe this is all about politics and fear and creating a climate. I have zero worries."

In Manhattan, architect Beth Miller lives and works near the New York Stock Exchange. "It's one of the ploys by the Republicans to take attention away from the stupidity of what they are doing," Miller said. "It's a media frenzy. You can get hit by an air conditioner falling out of a window."

The view of official government motives was no brighter in Newark, where vendors passed a sultry Sunday selling bootleg DVDs of "Fahrenheit 9/11" and "The Manchurian Candidate." "I don't believe it," Jamil Smith, a salesman, said of the terror alerts. "It's President Bush trying to win the election."

Toni Dolce, 25, lives in Lower Manhattan above Fraunces Tavern, where Gen. George Washington bade farewell to his officers after the Revolution. She and four friends split a $3,000 monthly rent and she digs the neighborhood. She shakes her head when asked whether anything would persuade her to leave.

"Only the second coming of Christ," she said. "We see the dregs of the earth on the subways and the sidewalks. We have such a nonchalant attitude."

Still, not all are so sanguine about the risk. Ben Eubanks, 29, a chef, lives in the Exchange, an old financial district office building converted into residential quarters. His parents called earlier Sunday, and he admits to a few jitters.

"Clearly, it's pretty discomforting given that [the Stock Exchange] is right across the street," he said. "If you live in New York, you understand this every day, the threat."

A few moments later, Frank Porro, a young computer consultant, walks out of the Exchange. He shrugs when asked about bombs. He knows a risk attends to life here.

"Everyone knows it's . . . a target since 9/11," he said, adding that his strategy is to "get out [of his apartment] early and come back late."

Porro had a friend visiting Sunday. "I told him to go do some sightseeing -- uptown."

Staff writers Dale Russakoff in Newark and Ben White in New York contributed to this report.

A security guard stands outside the New York Stock Exchange. Officials warned of possible terrorist attacks against iconic financial institutions.