Police bolstered their presence at high-profile locations. Extra officers with bulletproof vests, rifles and dogs were assigned to spots such as Grand Central Terminal in New York. Plainclothes officers handed out fliers at a nearby hotel with a warning in large block letters: "If you suspect terrorism, call the NYPD," the Associated Press reported.

The report said the warnings come amid an investigation centering on Najibullah Zazi, a 24-year-old Denver airport shuttle driver who authorities say received al-Qaida explosives training in Pakistan and was found entering New York City two weeks ago with bomb-making instructions on his computer.

Though Zazi is charged only with lying to the government, law enforcement officials said he may have been plotting with others to detonate backpack bombs on New York trains in a scheme similar to the attacks on the London subway and Madrid's rail system. Backpacks and cell phones were seized in raids on apartments Zazi visited in New York.

"It's not totally clear to us at this point what it is they had in mind, though I think it is clear that something very serious and something very organized was under way," said Attorney General Eric Holder.

"Several individuals in the United States, Pakistan and elsewhere" are being investigated, the FBI said.

"There's a lot more work to be done," said Police Commissioner Raymond Kelly, cautioning that the probe was still in its early stages.

In two bulletins sent to police departments Monday and obtained by the AP, federal counterterrorism officials urged law enforcement and private companies to be vigilant at stadiums, entertainment complexes and hotels.

The bulletin on stadiums noted that an al-Qaida training manual specifically lists "blasting and destroying the places of amusement, immorality and attacking vital economic centers." Counterterrorism officials are also advising police officers to be on the lookout for any possible bomb-making at self-storage facilities, noting that terrorists have used such places to build bombs.

The bulletins came just days after similar warnings about the vulnerability of the nation's mass transit systems and the danger of hydrogen peroxide-based explosives.

In a statement, the FBI and Homeland Security said that while the agencies "have no information regarding the timing, location or target of any planned attack, we believe it is prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."

New York's transit agency said it increased the police presence around the city. The vigilance is playing out during a meeting of the U.N. General Assembly, with Obama and other leaders from around the world in town. Also, thousands of policymakers and other visitors are arriving in Pittsburgh for a two-day economic summit of wealthy and developing nations.

New York's Police Department produced a 10-minute videotape it has begun showing at roll call instructing officers to be on the lookout for potential bomb-making ingredients. The video puts special emphasis on hydrogen peroxide — a common ingredient in homemade explosives — as well as cans of acetone and bags of ammonium nitrate.

Stadiums around the country provided few specifics about how they were responding but stressed that they have been vigilant ever since the Sept. 11, 2001, attacks.

"We're aware of the memo," said Bob Moore, spokesman for the Kansas City Chiefs, who play at Arrowhead Stadium. "It just underscores the high levels of security we've had and will continue to maintain. We've been in that mode for some time."

In New Jersey, home of Giants Stadium, the state homeland security office said there will be an increased police presence at key locations, random bag searches and greater use of surveillance cameras and undercover operations.

At MGM Mirage, which owns two arenas and the most casinos on the Las Vegas Strip, spokesman Gordon Absher said the alert "encouraged us to remain vigilant." No protocols were changed, he said.