The threat of a massive explosion at the the Washington Monument brought the heart of the city to a near halt yesterday, giving 20,700 government workers an unexpected holiday, disappointing thousands of tourists who could not get near many of the city's popular landmarks and infuriating motorists who were stuck in nightmarish traffic jams.

Fourteenth Street, which normally carries about 43,000 vehicles a day across the Mall, was closed, forcing outbound motorists to use the Ninth Street Tunnel or fight their way past the Lincoln Memorial. By mid-afternoon, both the tunnel and the Southwest Freeway were snarled. By 4 p.m., it was more than an hour's trip into the city from close-in Alexandria.

One man, emerging on foot from the tunnel, said, "I'm looking for my van pool. They're probably still back by the FBI . . . . "

Police initially closed the inbound-only 12th Street Tunnel under the Mall, but D.C. officials reopened it. "We're still trying to run a city here and we don't want to shut off everything," said traffic engineering chief Seward Cross.

Metro closed the Smithsonian station at 2:30 p.m.; trains continued to run through it, but did not stop. Rush-hour trains went into service earlier than usual and continued for an extra hour on the Blue and Orange lines. Escalators were shut off as a safety measure in stations handling unusually large crowds. Fifteen bus supervisors were put on overtime to try to reroute buses through downtown. The nighttime construction on the Woodrow Wilson Bridge was halted to help ease traffic problems.

Even planes taking off from National Airport were rerouted, with pilots taking off northward advised to keep to the west rather than following the Potomac. "The purpose is not to add to the confusion down there," airport spokesman David Hess said

The standoff affected the White House, where unspecified security measures were taken. President Reagan moved out of the Oval Office, which has windows facing the monument, late in the afternoon, officials said.

By 11:30 a.m., police, who estimated a blast radius at 1,200 to 1,800 feet based on the amount of explosives said to be in the van, had begun asking nearby government agencies to send their workers home. And by early afternoon, some 20,700 workers had been to told to leave their offices in the main Agriculture Department building and its four nearby buildings, the main Customs Service building and its bureau in the Liberty Loan Building, the main Commerce Department building, and seven of the eight Smithsonian Institution buildings on the Mall.

The first museum to close, about noon, was the Museum of American History, the closest to the monument, where officials were advised by police that "we could expect some minor damage: flying glass, broken windows, etc.," according to Ronald Colaprete, assistant chief of the Smithsonian's uniformed guards.

The National Gallery of Art, at the far end of the Mall, did not close.

Many of the bureaucrats, on their way to go Christmas shopping or drink a few beers at nearby establishments like the Old Ebbitt Grill -- and unaware that the day would end with the death of a man at the monument -- seemed more grateful for a break in the routine than concerned about any danger.

"We're discussing work, of course," joked a pinstriped Commerce Department lawyer, who used the free afternoon to watch "the Flintstones" on television with two friends at the Old Ebbitt Grill.

For thousands of tourists who had planned to visit the seven closed museums, which handle more than 30,000 people a day, it was a day of disappointment and instant change of plans -- from studying the sculpture at the Hirshhorn Gallery, for example, to gawking at the huge press stakeout on the Mall and trying to get a glimpse of the man threatening to blow up the monument.

Adrienne Matheson and Martin James, ballet dancers from New Zealand on their first trip to the U.S., had less than two days in the nation's capital and so headed straight for the monument. Though they were promptly turned back at Pennsylvania Avenue by police, the pair remained high-spirited, eager to take in this new American scene.

"Oh, I think it's great!" said James, 21, a soloist with the New Zealand Ballet. He and Matheson, a principal dancer with the same corps, considered the idea of returning to the monument today -- "if it's still there," James said.

As police began evacuating the Mall area about 2 p.m., the 48 second and third graders from Ridge Elementary School in St. Mary's County, Md., piled back on their yellow bus, having seen the Kennedy Center on their annual school trip but not much else. "First we thought it had something to do with the arrival of the Pakistani president," teacher Howard Hyman said. "Then we heard it was about some hostages."

"Some of the children want to know what a hostage is," said Debbie Dunbar, a teacher's aide accompanying the group.

Louis Doctor and Doug Walton, both young, successful computer specialists, were in town attending a computer graphics conference when they heard about the siege. "We decided to take the afternoon off and watch it the monument fall," said Doctor, 24, from Boston, as he and Walton hurried down 12th Street toward the Mall, where police were certain to tell them to turn around. "It's a good day to meet people. You can stand around and talk about the monument."

Kevin Fox, a 27-year-old airplane pilot from Springfield, and his friend Joanne Mullen, a 22-year-old airport service agent, had planned to spend their day off yesterday walking around Georgetown. But when they heard the news of the hostage situation, they decided instead to put on their roller skates and investigate.

It proved less exciting than expected, and the pair decided they might be better off in Georgetown, which Fox had never visited. "All you see are police," he said as he and Mullen skated away. "It's boring."

The incident aborted the book-signing session by artist Mike Major, with his recently published "Drawings of Washington and Georgetown," at the National Museum of American History.

The runners who have become a part of Washington's scenery moved serenely through it all, unstoppable. Dick Johnson, tall, lean and gray-haired, loped along the Mall in the late afternoon, when just about the only other people around were police officers and reporters.

"When you've been through a war, nothing stops you," he called behind him as he passed the Museum of Natural History. Johnson, who works at the Pentagon, said he wasn't worried about a possible explosion. "We're far enough away," he said. "All you need is 2,000 feet for a bomb."

And in the shadow of the 555-foot monument, the Bureau of Engraving and Printing was undeterred in its mission: Not once did its presses stop printing the nation's currency.