Even as police and government officials braced Monday for the possibility of a terrorist strike that authorities said could rival the Sept. 11, 2001, attacks, a nation busy preparing for the holidays appeared to take in stride this year’s fourth escalation of the national threat index.

Homeland Security Secretary Tom Ridge, speaking to reporters after a White House meeting on the new “code orange” alert, advised holiday shoppers and travelers to be vigilant but to stick to their plans.

“If you’ve got holiday plans, go. Don’t change them,” Ridge said. “We cannot be burdened by that threat or fear. We need to be alert to it.”

A major factor in the decision to raise the nation’s terror alert level from “elevated” to “high” was the holiday season, when more people are distracted and traveling and large numbers of people gather at events ranging from football bowl games to massive New Year’s celebrations in New York, Las Vegas, Los Angeles and elsewhere.

Yet the latest decision to go from “yellow” to “orange” on the terror risk scale was also based on specific, corroborated intelligence that al-Qaida may soon attempt to pull off a coordinated attack in multiple places to cause mass casualties — an attack that authorities said might eclipse that of 9-11.

“There are a number of credible sources that suggest the possibility of attacks around the holiday season and beyond,” said White House spokesman Scott McClellan. Such attacks, he added, are expected by terrorists being monitored overseas to “rival or exceed the scope” of those on Sept. 11 that killed about 3,000 people.

Several U.S. intelligence and law enforcement officials, speaking on condition of anonymity, said they had no specifics about a potential method, location or time of any attack. But they continued to point toward aviation as a prime possibility, noting that al-Qaida tends to return to what worked in the past.

Law enforcement officials have repeatedly also warned that al-Qaida might try to attack softer targets, such as malls or hotels, that have fewer security obstacles and which will be crowded this time of year.

Overt, covert security

Across the nation, authorities reacted to Sunday’s announcement by tightening security at bridges, tunnels, ports, landmarks, nuclear and chemical facilities, and other possible targets of attack. The police presence on New York’s streets and in its subway system increased visibly. The FBI established 24-hour command centers at all 56 of its field offices and security at the United Nations was tightened.

Robert Bonner, commissioner of the Customs and Border Protection Bureau, said his agency has boosted inspections at all 301 U.S. entry points. Holiday leaves were canceled for Customs and Border Patrol agents.

“We are increasing our scrutiny of both people and vehicles coming into the United States,” Bonner said.

The North American Aerospace Defense Command (NORAD) said it had stepped up air patrols with F-16 fighter jets around “critical infrastructure.”

A layer of protection at airports was likely put in place over the past 24 hours that is not outwardly apparent, security experts said. Among the likely steps were an increase in the number of air marshals, particularly on flights arriving from overseas, and undercover surveillance around airports, said Brian Jenkins, research associate at the Mineta Transportation Institute at San Jose State University and a special adviser to the Rand Corp.

Federal officials would not discuss their planning at such a detailed level.

Fears not elevated

Homeland Security officials, concerned that the public had begun to ignore their warnings, had vowed earlier this year to avoid raising and lowering alerts frequently. Three orange alerts were sounded in just four months earlier this year. Each time, the alert status is raised from yellow to orange, it costs government agencies across the nation $1 billion a week, according to David Heyman, director of homeland studies at the Center for Strategic and International Studies, a Washington think tank.

“They really go up and down on those things, don’t they?” said Catherine Behiling as she waited with her three grandchildren to have their photos taken with Santa Claus at a Santa Monica, Calif., mall. “I don’t think people pay attention to it. I haven’t noticed anything different when they’ve done it before and I don’t notice anything different now.”

And in downtown Miami, Marcel Moellenbeck, 24, had no intention of changing his plans for a trip to Disney World because of the threat level. “We have no fears of flying,” said Moellenbeck, a student from Germany. “The chances of something happening are really, really low. I believe it’s relatively safe. Everywhere could be a terrorist’s target.”

Travel advisory

People traveling by airplane are urged to give themselves extra time to get through airport security. The Transportation Security Administration has a travel tips bulletin on its Web site, www.tsa.gov.