For the first time since the dark days of World War II, the White House closed its doors to visitors yesterday because of security concerns, 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 has been a growing number of anti-war demonstrations outside the White House and said some protesters had tried to climb over the fence.

Almost 100 people have been arrested outside the White House in recent days, including two men who threw body bags on the lawn of the historic landmark.

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

The White House, the oldest public building in Washington and the home and office for every president except George Washington, was last closed for security reasons on June 6, 1941, according to a White House and National Park Service spokesman. It reopened in November 1946, after World War II ended.

During the Vietnam and Korean wars as well as the Cuban missile crisis and other international crises, the popular tour -- a symbol of the nation's access to its leaders -- remained open.

"It's the people's link to the government. It gives them assurance to have it open," said William Seale, author of "The President's House," a history of the White House. "It is the government to us."

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 a different route 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 U.S. Capitol is the tightest it has been in years. Uniformed and plainclothes police and explosive-detecting dogs patrol the halls with increased frequency.

Federal law enforcement officials said this week that they have foiled more than five terrorist schemes in the United States since Iraq invaded Kuwait on Aug. 2.

"Some were plans headed off, some were individuals attempting to concoct plans," one official said. The cases involved "individual zealots," not terrorist groups, the official added.

A Kuwaiti-born American was arrested in New Jersey on Nov. 29 on charges of conspiring to kill President Bush and other federal officials. In other cases, investigations continue or the suspects left the country.

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.

Staff writers Barton Gellman, Sharon LaFraniere, George Lardner Jr., Donnie Radcliffe and Al Kamen contributed to this report.