More than 300 anti-nuclear demonstrators were arrested at the Pentagon yesterday and hundreds more rallied in their support during a massive symbolic effort to "shut down" the huge Defense Department headquarters in suburban Virginia.

Shouting "No more nukes!" and "Shut down the war machine!" the protesters jammed entranceways, blocked traffic at some points and splattered what they said was blood against the building.

To no one's surprise, the attempt to close the Pentagon failed. But hundreds of employes were inconvenienced or slowed as they clambered over and around the chanting protesters at the entrances and heard their pleas to stop working for the "death machine."

The avowedly illegal "civil disobedience" action marked the largest mass arrest of political demonstrators on a single day here since the Mayday anti-war demonstrations of May 3, 1971, when more than 7,000 persons were arrested in downtown Washington.

Hundreds of helmeted Federal Protective Service police, in scenes reminiscent of the Vietnam antiwar days a decade ago, were kept running throughout the day-long protest yesterday, arresting demonstrators, clearing roadways and attempting to keep the Pentagon's many entrances open.

Demonstrators, many in faded blue jeans and carrying knapsacks and bedrolls, chanted antinuclear and antiwar slogans. Some linked arms and blocked Pentagon employes attempting to enter the building. Police with clubs drawn shoved protesters back, sometimes roughly. Other protesters let the air out of official cars and blocked roadways with trash barrels, boxes, concrete blocks and anything else they could find.

Those arrested were the shock troops of the new antinuclear movement -- mostly young, white middle-class youngsters much like their predecessors of the Vietnam War era.

But some veterans of that era were also arrested yesterday: baby doctor Benjamin Spock, radical pacifist David Dellinger; Pentagon Papers figure Daniel Ellsberg; longtime antiwar priest Philip Berrigan.

Spock, now 77, raised his fist to cheers as police dragged him from the river entrance into the Pentagon.

"We can't nonviolently close down the Pentagon," Ellsberg said. "That's not our aim. But we can show we have the freedom to risk our freedom -- the risk of going to jail."

Ellsberg, who was a GS-18 Pentagon official before he soured on the Vietnam war, looked up at one of the windows above the river entrance and said, "I was standing there in 1967, watching them arrest demonstrators.

In another trenchant reminder of the Vietnam era, John David, Borgman, who identified himself as a former U.S. Marine Corps fighter pilot, burned his captain's uniform on the river entrance plaza, weeping with his hands to his face as scores of demonstrators looked on.

Apparently spurred by recent events in Iran and Secretary of State Cyrus Vance's resignation, the turnout at yesterday's protest was larger than organizers had expected.

The Coalition for a Non-Nuclear World said it had planned on about 500 demonstrators, but police estimated the crowd at 1,200.

While hit-and-run civil disobedience activity went on most of the day, it peaked twice -- first when the protesters arrived around noon, then again between 4 and 5 p.m. when those who still had not been arrested attempted to disrupt Pentagon employes as they poured out of the building to go home for the day.

Below ground, about 100 demonstrators each blocked the two main ramps leading from the Pentagon concourse to the Metro subway station. Police ripped apart the lines of protesters linked arm-and-arm so employes could pass through.

Noting that few arrests were made at this point, Federal Protective Service operations chief John Jester said, "We're just trying to get the employes out and let things cool down."

By day's end, Pentagon officials estimated 300 to 400 demonstrators had been arrested at various entrances. Most were charged with obstructing entranceways to a federal building. Some were taken before U.S. magistrates in Alexandria who imposed penalties ranging from $5 to $50 and one to five days in jail.

Earlier in the day the demonstrators gathered across the Potomac River at the Department of Energy, 10th Street and Independence Avenue SW -- another target of the antinuclear movement.

Protesters leafleted employes as they entered the building and began forming small "affinity groups" for the siege of the Pentagon later, Helmeted police guarded all DOE doors.

A procession of chanting protesters snaked through the crowd, led by a blacked-robed "death" figure and several persons in white nuclear technician suits and surgical masks carrying a black coffin.

"Can't stop the spirit -- she's like a mountain -- old and strong -- she goes on and on," chanted the protesters as curious DOE employes looked down from their windows above. A few employes occasionally waved to the demonstrators.

"We're here," said Ellsberg, "because the policies of both these buildings [DOE and the Pentagon] have been most unwise, and their dangers have never been so clear."

At about 9:45 a.m., the demonstrators -- most of them schooled in civil disobedience tactics at special training sessions last week sponsored by the coalition for a Non-Nuclear World -- started marching toward the Pentagon.

They split into three groups to get to the Pentagon: about 635 taking Memorial Bridge, 525 on the 14th Street Bridge and a smaller number on the Metro subway.

Slowly, the colorful crowds with huge anti-nuclear banners billowing over their heads converged on the Pentagon. Once on the Virginia side of the river, they broke into still smaller groups and scattered to all five main entrances around the low-lying Defense building.

Harried Federal Protective Service police officers began dragging protesters away, sometimes yanking them roughly as they brought them inside the Pentagon for processing.

Other protesters waiting to be arrested sang "We Shall Overcome" and tried to speak to the federal police officers. "We love you . . . We don't want to hassle you."

As soon as one group of protesters was removed from an entrance, a new group would take its place. Some protesters sat unarrested for up to a half hour or more as short-handed police scrambled to keep arrest teams available.

Many demonstrators came to protest but not to be arrested. They attempted to stop ingoing Pentagon employes and engage them in sidewalk debates on the pros and cons of nuclear weaponry.

Some ignored the demonstrators or snapped at them angrily. Others stopped to talk politely.