The day the United States attacked Iraq, the fear of terrorism brought change to Washington: tighter security at high-profile installations, a ban on curbside baggage check-in at airports, no public tours of the White House for the first time since World War II.

But in some other ways, life was expected to go on as usual. The Smithsonian museums and the city's public monuments were expected to be open today. Metrorail and Amtrak planned normal schedules. Local authorities said they planned no highway closings for security reasons.

Fearing terrorism, the White House closed its doors to visitors yesterday even before the attack began, indefinitely canceling the public tours that normally attract 3,500 people a day.

"The tours offer the most immediate opportunity for unwarranted intrusion into the White House," White House spokesman Marlin Fitzwater said just hours before U.S. bombs started falling on Kuwait and Iraq. "As long as we have any threats to the physical building of the White House, prudence indicates we shouldn't have any tours."

Fitzwater noted that there have been a growing number of anti-war demonstrations outside the White House and said some protesters had tried to climb over the fence. More than 100 people have been arrested outside the White House in recent days, including two men who threw body bags on the lawn of the mansion.

The State Department and the Pentagon also halted their public tours yesterday as security precautions were visibly increased throughout Washington government and military buildings.

Passengers arriving at Dulles International and National airports today could expect to see more prominently posted security guards, and they may experience delays, although no flights were canceled, officials said.

At 8 last night at major airports nationwide, the federal government imposed what it calls "preplanned security level 3," one step below the most stringent security requirements. Under the security plan, more uniformed police are moved into airports and a more stringent "profile" is applied to passengers in determining whom to question and search.

In addition, access to airports is tightened and gate areas are limited to ticketed passengers. More involved searches, and long lines, are likely at security gates.

The higher security was ordered although "no specific, credible threat has been received of terrorism against any airline," a government source said.

Travelers at Dulles and National had to check their own luggage yesterday because curbside check-in was suspended.

At Dulles, passenger lines backed up as guards took extra care to check bags through security and X-ray machines. Even airline personnel and pilots, who normally have free access, had their identification checked more closely before being allowed through metal detectors. Dulles employees said trash cans and ashtrays were removed from some public areas.

But Joseph Hurtuk, National's deputy police chief, said there were no indications that air traffic would be disrupted. At Dulles, Henri Cloutier, airport operations officer, said the airport expects no delays in international flights.

Institutions ranging from the Smithsonian to the region's nuclear plants -- Calvert Cliffs in Lusby, Md., and Virginia's North Anna, about 75 miles southwest of Washington -- tightened security in response to potential terrorist threats, but spokesmen would not specify what measures were taken.

"We are constantly in touch with all the agencies that advise us on security measures," said Madeline Jacobs, spokeswoman for the Smithsonian Institution. "I mean constantly, like every 15 minutes."

"We feel we're prepared for any terrorist activity," said Peggy Mulloy, spokesman for Baltimore Gas & Electric Co., which operates Calvert Cliffs.

White House security officials, who began checking the identifications and parcels of even high-ranking administration officials yesterday, also closed off East Executive Avenue to traffic.

The First Lady's spokesman, Anna Perez, who had her purse examined yesterday as she entered the White House, said the tour shutdown "would be reviewed periodically" to determine when the tours will resume.

In other security precautions, the drivers of some high-ranking Pentagon generals are being armed, and they are taking the officials to work by different routes each day, according to Pentagon spokesmen.

Lawmakers and congressional aides also have been advised to vary their daily routines and their routes home, to use the network of tunnels at the Capitol whenever possible and to remove obvious congressional identification from their automobiles.

Security at the Capitol is the tightest it has been in years. Uniformed and plainclothes police and explosive-detecting dogs patrol the halls with increased frequency.

Asked about security arrangements, a Pentagon receptionist talking to a reporter said in mid-conversation: "I have to go. We have a bomb threat. I have to leave the room."

In a later conversation, she said three floors of one small sector of the building were evacuated but that there was no bomb.