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 Sept. 11, 2001.

"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.

"We know, tragically, they turned four airplanes into missiles," Homeland Security Secretary Tom Ridge told reporters after a meeting of Bush's Homeland Security Council, which recommended the change in terror threat level on Sunday.

Cargo planes and flights originating overseas were of special concern. Officials say steps have been taken to improve security in both areas, but some critics call for the screening of all packages on cargo planes to prevent smuggling of bombs.

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 told The Associated Press in an interview. "I believe we will be able to increase to correspond to level orange without undue delays."

A particular focus, Bonner said, is screening of cargo containers at seaports in the United States and abroad.

Defense Department officials said they were launching more military air patrols over major cities, but they would provide no details. Other government sources said, however, that New York, Washington and Los Angeles were likely choices.

The State Department advised all U.S. embassies of the change in terror alert level and had previously issued a worldwide caution about recent terror attacks in Saudi Arabia, Turkey and elsewhere. Spokesman Richard Boucher said threats in those places were not diminished by the increased risk of attack within the United States.

"We have been concerned and need to be concerned worldwide because of the nature of the group that we're dealing with," Boucher said. "Al-Qaida has worldwide capabilities."

The United Nations put its headquarters staff in New York on high alert as well. The U.N. was recently the target of a deadly terrorist bombing in Baghdad that killed a senior official.

The Bush administration coupled its message of extra vigilance with an appeal to Americans not to abandon plans to travel, even if airports presented more of a hassle because of increased vehicle and baggage searches, police with bomb-sniffing dogs and parking restrictions.

"If you 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."

People were urged to give themselves extra time to get through airport security. The Transportation Security Administration put a travel tips bulletin on its Web site (www.tsa.gov).

Another layer of protection was likely put in place over the past 24 hours that is not outwardly apparent, security experts said. Among the likely steps was 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.

Most travelers appeared to take the restrictions in stride, several airport officials said.

"I'm always impressed with passengers' ability to respect security concerns. The mood is very supportive," said Fred Szabo, commissioner of Cleveland Hopkins International Airport.

Across the country, security was strengthened for key bridges, tunnels, seaports and landmarks, as well as nuclear and chemical facilities and other installations that might be vulnerable to attack. At the Gateway Arch in St. Louis, officials said heightened security would include increased patrols and police vehicles.

"You hate to see that, but that's part of our plan," said Ken Schaefer, superintendent of the arch.

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.