Major cities and transportation hubs around the nation increased security patrols Sunday in response to the raising of the national terror alert to its highest level in months.

The heightened alert prompted heavier security at buildings ranging from nuclear plants to shopping malls, and was expected to cause delays at many of the nation's airports and border crossings.

Governors across the country offered the same basic message: Although residents should be vigilant, there was no specific threat against their communities and they should stick to their holiday plans.

"We encourage people to go about their lives. I hope this is yet another false alarm, but we have to be prepared," said Massachusetts Gov. Mitt Romney. He returned to Boston from a family vacation in Utah to deal with the heightened alert, but planned to fly back Monday.

Many shrugged off the warning.

"They're like earthquakes. You learn to deal with it," said 42-year-old Jeff Shaw of Reno, Nev., during a family trip to the San Francisco Shopping Mall.

Even those who said they were nervous didn't think the alert would change their plans.

"What are we supposed to do differently? Either they're going to bomb us or they're not," said Curtiss Jacobs of Lafayette, Calif., who was meeting friends for lunch in downtown San Francisco (search). "You just have to live your life."

Federal Homeland Security Secretary Tom Ridge (search) announced in a hastily arranged news conference Sunday that he was raising the national threat level to orange, the second-highest level, saying attacks were possible during the holidays and that threat indicators are "perhaps greater now than at any point" since Sept. 11, 2001.

Orange means a high risk of terrorist attack. Since May, the level had been at yellow, or an elevated risk, and in the middle of the five-color scale.

Ridge cited reports that Al Qaeda, Usama bin Laden's terrorist network, is trying to find holes in U.S. aviation security, and that "extremists abroad" are anticipating attacks that will rival or exceed the scope of the terrorist attacks of Sept. 11, 2001.

Ridge also spoke about the increased terror risk to governors and other state officials in a conference call Sunday.

Officials are unaware of a specific target or means of attack, but some of their intelligence mentions New York, Washington and unspecified West Coast cities, said a senior law enforcement official who spoke on condition of anonymity.

New York City Police Commissioner Ray Kelly said his department was putting more officers on the streets, establishing checkpoints at bridges and tunnels and patrolling the waterways.

Mayor Michael Bloomberg said that although there were no specific threats, "we have to always act as if there are because it's the best way to deter a terrorist attack."

"Our great strengths are what make us the obvious target," Bloomberg said.

The Golden Gate Bridge and other San Francisco Bay-area spans now have beefed-up patrols, undercover officers and mandatory checkpoints for trucks, said Sgt. Wayne Ziese, division spokesman for the California Highway Patrol. Areas around oil refineries and nuclear facilities also are getting extra attention, he added.

The FBI's Joint Terrorist Task Force (search) in Philadelphia set up a command post to receive and check out tips, said FBI spokeswoman Linda Vizi. Residents who hear or see something suspicious should call 911 if they think a threat is imminent or the FBI if it's something that can wait, she said.

"People shouldn't determine in their own mind whether they think it's significant, they should let us take a look at it," Vizi said.

Patrols were increased immediately at Florida's Port Everglades at Fort Lauderdale, where some 50,000 passengers were on 15 cruise ships, port spokeswoman Ellen Kennedy said.

At Boston's Logan Airport, where the two hijacked planes that hit the World Trade Center on Sept. 11 originated, officials added more state police at curbs, terminals and along perimeter roads Sunday, Logan spokesman Phil Orlandella said.

Many airports resumed or were planning to resume random vehicle searches, including those for Dallas-Fort Worth, Miami, Minneapolis-St. Paul, Philadelphia and San Francisco. In some cases, all cars will be searched, said Doug Wills, spokesman for the Air Transport Association, the airline industry's main trade group.

Travelers were advised to arrive at airports an hour earlier than usual to get through the additional security.

Border officials said the heightened alert combined with the holiday season probably will mean longer waits for people entering the United States.

"We will search more vehicles and more trunks of vehicles," said Mike Milne, a U.S. Customs Service spokesman in Washington state. That doesn't mean U.S. travelers should not buy Christmas gifts in Canada or vice versa, he said -- "just don't wrap them."

Emergency officials in California and other states said they may increase security measures even further, but were waiting for more detailed information.

"We have gone through these many times, and we don't want to ramp up the full operation and find ourselves sitting there waiting and wasting a tremendous amount of money," said Dale Chessey, a spokesman for the California Office of Emergency Services.

Many Americans reacted to Ridge's announcement with a mix of resignation and defiance against terrorists.

"Would it stop me [from holiday travel]? Absolutely not. I have no fear," said Michael Patrick McCormack, a retired fire lieutenant in Brooklyn. "You just can't prepare for everything. To crawl in a hole -- no pun intended for the butcher of Baghdad -- but that's what he had to do, not us."

The alert came at a bad time for the nation's retailers, who are hoping for a strong end to what has been an uneven holiday shopping season. Major mall operators such as Taubman Centers Inc., which owns and manages 31 shopping centers in 13 states, immediately stepped up security.

John Courtney, a Bostonian standing by the tree in New York's Rockefeller Center on Sunday, said the alert made him "maybe just a little more cautious."

"You can't stop your life because of this. That gives them exactly what they would want," Courtney said.