On Capitol Hill yesterday, the imminence of war jolted the complacent.

Capitol police halted each vehicle cruising into the underground garages, ready to turn away any occupant who could not produce a staff pass. At public entrances, security was doubled, and the same guards who used to wave the familiar on through were dutifully checking ID tags. "An old ploy," explained Sen. Alan Simpson, "is to get someone trusted and send a bomb in with them." And at special briefings for both the House and the Senate yesterday, the CIA and FBI were encouraging a new vigilance.

"Oh, you can be sure," said Simpson (R-Wyo.), "it's all very real."

With the United Nations deadline for Iraq's withdrawal from Kuwait just hours away, official Washington was bracing for the worst. From the monuments to Metro, from the Hill to the corridors of the Fourth Estate, Washington found itself with a citywide case of the jitters.

The fear stems from the capital's potential as an obvious magnet for a terrorist attack. As one police officer said several years ago: "This city is the symbolic capital of the world. If you want to strike a blow against the United States of America, you don't go to Des Moines and blow up a cow."

Accordingly, most federal buildings and agencies, as well as high-profile public institutions, went on red alert.

The National Park Service, for instance, increased patrols around the national monuments, routinely considered targets. Sandy Alley, spokeswoman for the service, said that guards have also been instructed to be "more vigilant about the kinds of people entering the monuments."

At about 5 p.m. yesterday park police were alerted to a man with a briefcase reportedly running down a street near the Kennedy Center saying he had a bomb. They said they were looking for him but had no detailed description.

At Metro, spokeswoman Beverly Silverberg said that while the transit system isn't taking any special measures, "we're always on the lookout" for potential threats. "Our transit police are trained with the FBI. ... We work with them on an ongoing basis."

Smithsonian Secretary Robert McC. Adams said yesterday that there were "contingency plans" for the federally run museums in the event of conflict, but declined to say what they were. "It's important not to have anything that looks like saber-rattling," he said. In 1986, fears of terrorism in Washington prompted the Smithsonian to close its underground garages to the public. They have never been reopened.

"We try not raise the profile of security," explained Madeleine Jacobs, spokeswoman for the Smithsonian, "but our guards are certainly aware of the situation in the world."

The national media, too, nervously looked over their shoulders. The Wall Street Journal's Washington bureau got a call at 3:55 p.m. from someone who said "they were going to blow us to pieces at 4:30," a newsroom employee said. Police were called and the staff evacuated the building but nothing was found.

The caller, who spoke with an accent that was not readily identified, did not give his name and offered no reason for the threat.

At Progressive Technologies of America in Falls Church, a major manufacturer of bulletproof vests, President Terry Arnold reported that journalists have been big customers of late. "We prefer to sell to organizations that have some connection with law enforcement," said Arnold, who added that his business has nearly doubled in the past two weeks. "An individual has to look me in the eye and convince me he's responsible before I sell him one."

And at the National Press Building, home to most of Washington's mammoth media contingent, tenants were presented with a cold dose of reality Friday in the form of a memo from the building managers. For one, they were told, use of the two elevators that are outside the view of the guards was abruptly terminated. "We are a well-known public building just two blocks from the White House," William D. Hickman, president and CEO of the National Press Club Building Corp., explained yesterday. "It occurred to us that we needed to be prudent. We beefed up security. ... We wanted to make sure that anyone entering the building could be seen by our guards."

Still, nowhere was security more evident yesterday than throughout the halls of Congress, where Capitol police planned stepped-up protection for their 535 wards. Police spokesman Dan Nichols said that Hill security was tightened the day the Geneva talks between Secretary of State James Baker and Iraqi Foreign Minister Tariq Aziz collapsed. "You have to remember these are public buildings," Nichols said, "and all I can tell you is that free access and security are definitely opposing interests."

"Even the members' cars are being stopped when you try to drive onto the Capitol plaza," said Rep. Dennis Eckart (D-Ohio). "I was taken aback. I mean, these guards know us -- they're trained to memorize our faces -- and they're still stopping us. You see it everywhere -- in doorways, the halls. They're even bundling the mail smaller to be able to scan it for bombs. Security is much more visible."

"It was sort of eerie," said one lobbyist who visited the Hill yesterday. "I had to send my coat through the X-ray machine, which I've never had to do before."

Said Sen. Simpson, who attended the security briefing yesterday: "I'm not a worrywart by nature. I've had my life threatened before, and it's not pleasant. But all this does make you take pause and call a loved one like your wife, and tell her to be careful about strange-looking packages. This has been very disconcerting for us."

Staff writer Martin Weil contributed to this report.