Eighteen antinuclear-power demonstrators were arrested here yesterday following orchestrated acts of nonviolent civil disobedience at the beginning of a planned series of scattershot assaults on the controversial Seabrook nuclear power plant.

State police dragged and carried the protesters to a waiting bus to be booked and arraigned in nearby Hampden as construction workers returned to work for the first time in three weeks.

Most of the demonstrators arrested refused to post bail, and officials began looking for county jails large enough to hold them.

The plant was shut down July 21 by the Nuclear Regulatory Commission pending approval of the facility's oceanwater cooling tunnels by the Environmental Protection Agency. EPA approval came Aug. 4, and the NRC voted last Wednesday to allow construction to resume.

The demonstrators protesting that decision are members of the Clamshell Alliance, a coalition of New England antinuclear-power groups. Early yesterday morning, they divided into three groups of six each and staged a series of events to dramatize their opposition to the plant after this latest defeat before the regulatory boards.

One group of demonstrators shackled themselves to a large crane on the construction site and mounted a banner that read, "No nukes." The second group of protesters chained themselves to a sign by the plant's front gate reading "Seabrook Station." Below the sign they tacked a placard that read, "Will not be built."

A third group walked through an opening in the chain link fence and sat along the edge of the service road. A security guard, reading from a plastic card, warned them that they would be arrested for trespassing. State troopers immediately moved in for the arrests. Large bolt cutters were used to release the other protesters so they, too, could be arrested.

"They should let the workers take care of those b -," shouted Edward Smith, one of 1,800 people laid off during the work stoppage.

"They oughta get this trash outta here; it stinks," said another.

Gordon McKenney, a spokesman for Public Service Co. of New Hampshire, builders of the $2.3 billion plant, said that no work time was lost because of the demonstrations. He called the protest "part of the continued harrassment by the Clamshell Alliance."