When Pierre L'Enfant designed the city of Washington, he structured it with wide boulevards and traffic cirlces so that it could not be easily tied up by violence, as Paris had been during the French Revolution.

Yesterday, it was obvious that L'Enfant failed. All anyone has to do to bring Washington to a standstill is seize the District Building, the B'nai B'rith offices on Rhode Island Avenue and the Islamic Center on Massachusetts Avenue at Waterside Drive.

For most of the day, while the city glowed under a warm, bright March sun, traffic snaked and tangled in mile-long snarls, clotting around the three areas where gunmen held a total of between 20 and 100 hostages.

The institutions of Washington, large and small, reeled under the influence of a handful of men. The White House switchboard was swamped with calls from around the country, the Washington Monument was closed to visitors as an antisniper precaution, and an official state visit from the prime minister of Great Britain, was almost called off, according to a Secret Service agent on duty at the Islamic Center mosque.

At the Tiffereth Israel Synagogue, at 7701 16th St., across from the local headquarters of Hanafi Muslims, Hebrew School was canceled for the afternoon.

Washingtonians themselves, drunk on the warm weather and excitement, miled through the streets and around the barricades like patrons of some surreal guerrilla theater.

"These people must be insane," said a young woman standing near the police-encircled B'nai B'rith building. "But this beats going back to work."

While the barricades surrounding the gunmen-held buildings were crowded with reporters and onlookers, and the streets were crammed with cars, the buildings themselves appeared to be islands of calm for most of the day.

Police closed normally crowded Massachusetts Avenue for two blocks on either side of the Islamic Center and, at midafternoon, the mosque itself towered in unaccustomed silence while police tactical forces sat on the steps in the sun.

They wore blue flak vests but no helmets and joked with each other while other officers milled around the back of the mosque with riot guns and a police marksman crouched with a rifle behind a tree across the street.

At one point, a woman dressed in a sari wandered into the middle of the street in front of the mosque and smilingly explained to a startled police officer that she had left her car parked next to the mosque. She walked past the windows of the mosque, got in her car and drove away.

Beginning at midday in the heavily-traveled downtown area, the hostage incidents triggered the kind of gluey, horn-honking traffic jams for which Washington is justly infamous.

With the city's "man street" - Pennsylvania Avenue - and three major arteries for suburb-bound traffic cordoned off, both automobiles and buses were detoured onto other, already burdened, streets.

Pennsylvania Avenue buses were detoured onto F Street. Virginia-bound buses were detoured from 14th Street onto 11th Street and Constitution Avenue to bypass the District Building. Massachusetts Avenue buses that normally pass the mosque were sent across the Taft bridge and onto Calvert Street.

Eastbound traffic that usually goes our Rhode Island Avenue was shifted southward to L Street, where it was bumper-to-bumper as far as one could see. Hundreds of commuters jammed I Street sidewalks at 19th Street, waiting for scores of buses, locked three abreast in a traffice standstill to take them to Maryland and Virginia.

Illegally parked cars and stalled vehicles made the traffic situation even worse.

Metro public relations spokesman Cody Pfanstiehl said many bus lines were running "15, 30 minutes, even an hour or more behind schedule."

Pfanstiehl said Metro was advising passengers to "walk along your bus routes until, hopefully, you might come to a point where the buses are unstuck and can get you home."

Hundreds who took that advice and got to the Bureau of Engraving, near the approach to the 14th Street bridge, to Virginia filled the sidewalk waiting for their ride home.

For a long time, no buses came, because some Metro drivers - independently seeking the quickest way to reach the bridge - went onto the Southwest Freeway and bypassed the Bureau of Engraving bus stop.

At the White House, Presidential Assistant Hamilton Jordon monitored the situation from the beginning. But the accuracy of information inside the White House was at times no better than it was outside.

A mid-afternoon Jordan got reports that Councilman Marion Barry was dead and Mayor Walter Washington was being held hostage. Both reports were erroneous.

There also were rumors among onstation patrolmen that the terrorists might plan a rush on the White House. No one ever seemed to have enough information.

At the Capitol, security virtually was doubled from the normal day shift complement of 220 to 250 uniformed officers to at least 440, according to District Police Capt. Harry Grevey.

"We're asking everyone to be very careful," he said.

The violence had a domino effect that stretched far beyond the buildings that contained it and even the city itself.

In mid-afternoon, for example, the Secret Service was concerned about more than the hostages. The gunmen in the mosque on Massachusetts Avenue were within firing range of both the Vice President's residence on Observatory Circle and the British Embassy.

In the air over the Atlantic at the time were British Prime Minister, James Callaghan, and several members of his cabinet, en route to Washington by Concorde supersonic jet for an official state visit. The British flag was flying in honor of his visit just outside the District Building where the bullets also were flying.

It was too late to turn them back. The plane landed uneventfully and Callaghan was driven to Blair House, the residence for visiting dignitaries across from the White House.

Telephone calls from around the nation and around the world flooded phone circuits in the metropolitan area.

"Everyone is calling Washington to see about their families," said one harassed operator patching in a long distance call.

A Washington Post operator, answering the paper's main switch-board number, was informed by the caller that she was speaking on Australian radio and could she please describe for the Australian people what was happening in Washington.

The flood of calls triggered a telephone company recording urging callers trying to reach an operator to make only urgent calls and to try to dial them direct.

Meanwhile, in the calm, residential neighborhood at 16th and Juniper Streets in far Northwest Washington, a guard dressed in a T-shirt and blue jeans, a machete at his belt, paced before the dark, handsome Tudor-style house that is the local Hanafi Muslim headquarters.

There were no police cars in sight. The only sound, nearly obliterated by the muted roar of homebound cars on 16th Street, was the soft flapping of Muslim and African flags flyiny from six flagpoles in front.