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 are prepared and as safe as we can possibly be,” Connecticut Gov. John G. Rowland said. “Leave the worrying to us. That’s our job.”

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. “You just have to live your life.”

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

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.

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 Assn., 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.