Fifty-six antinuclear demonstrators were arrested yesterday when they refused to move away from the entrance to the Nuclear Regulatory Commission downtown during a dramatic "die-in" protest against construction of the controversial Seabrook, N.H., nuclear power plant.

Screaming and collapsing in a pile of tangled bodies on the sidewalk in front of the NBC in mock reaction to a lethal "nuclear accident," the protesters were hauled away by D.C. police as a large noonday crowd of secretaries, businessmen and other curious onlookers watched.

The action, supported by some 200 singing and chanting protesters who have been camped out on the steps of the NRC at 1717 H St. NW since Wednesday, brought the nation's escalating nuclear power controversy to the streets of Washington for the first time on a large scale.

The protesters - many of them veterans of earlier mass demonstrations at the nuclear plant site in Seabrook - are here awaiting an NRC decision which is expected today on whether construction of the Seabrook plant should be suspended while further technical and environmental studies are done and possible alternative sites are considered.

Organizers indicated they will stay at the NRC at least until the decision is handed down and may stage another sidewalk "guerrilla theater" protest similar to yesterday's, especially if the NRC decision permits construction of the plant to continue.

The demonstrators arrested yesterday were charged with "incommoding the sidewalk" under the city's disorderly conduct statute and held under $50 collateral each.

The protest was an exercise in civil disobedience - deliberate violation of the law to dramatize a political point.

Organizers notified police of the carefully planned "die-in" scenario several hours in advance. At 12:35 p.m., the die-in started with three protesters acting as NRC "commissioners" announcing a "nuclear accident." Protesters, some wearing death masks, began screaming, swooning and then collapsing on the sidewalk, their bodies blocking entry to the NRC building.

Deputy Police Chief Robert Klotz, commander of several squads of officers assembled at the scene, told the protesters over a bullhorn that they would be arrested if they did not leave. There was no movement by the "dead" protesters as some 200 supporters at the edge of the scene began singing antinuclear songs. Police teams moved in, arresting the "dead" protesters ony by one and carrying them to nearby paddy wagons. The arrest procedure took about an hour.

"Swing low, sweet chariot," sang the protest support group quietly. Then later, to the tune of "Down by the Riverside," they sang, "We're gonna lay down the Seabrook nuke, down by the NRC."

"The NRC is the Pentagon of the 1970s," said Chuck Matthei of the Seabrook Natural Guard, principal organizing group of the protest.