The Sandwich Clams, the Wind Clams, the Freshwater Clams, the Charles River-Antinuclear Kite-Flying Society, the Spider Warts, the Elliott Solar Cookers, the Portland Pine Cones, the Solar Rollers, the Sisters of Silkwood and the Pelecypods set up camp yesterday on the edge of this small costal village.

Numbering 2,500 to 3,000 yesterday, they carried to a portest site here colorful banners and placards proclaiming: "No Nuke Is Good Nuke," "Nukes Kill, Plug Into the Sun," "Nukes Kill Children" and "No Nam, No Nixon, No Nukes."

Drawn from across the nation, they represent what had become the cutting edge of the antinuclear power movement, but they were a mellow group yesterday, more interested, as the old antiwar slogans says, in "making love, not war."

The members of Nukus Interruptus and Clam Haven I were in the woods rubbing one another's backs. They do this sort of thing often, said Paula Goodman of New Haven, Conn. "It's a matter of togetherness. There are a lot of things we do just to give us a feeling of community. To get to know and trust one another better."

Nukus Interruptus and Clam Haven I are "affinity groups" - demonstrators organized in clusters to feed and shelter each other during this protest and to protect one another if trouble breaks out.

Some of the affinity groups have been training for months for a planned three-day antinuclear demostration here this weekend that organizers claim will be the largest of its kind in the nation. The demonstration is being held on the grounds of a $2.3 billion nuclear power plant that, if completed, will also be the largest in the nation.

By late yesterday afternoon, the demonstrators had erected a colorful tent city on the demonstration site, located at the former town dump.

A day-long rally is planned for today featuring a host of personalities from the antiwar movement, including Dr. Benjamin Spock, Pete Seegar and Arlo Guthrie.

More than 1,400 people were arrested on trespass charges in the last demonstration here in May 1977. Even more arrests had been expected this year.

However, responding to pressure from area residents, the Clamshell Alliance, a coalition of anitnuclear-power groups, recently agreed to avoid direct confrontation with police and to restrict the three-day demonstration to a 18-acre area.

This caused a severe rift among those in the alliance who had been training in civil-disobedience techniques and had planned to be arrested to dramatize their opposition to the nuclear power plant.

Donna Vega, a Clamshell activist from Boston, accused the group of "selling out" to the state of New Hampshire in agreeing to hold a peaceful demonstration.

"It's beginning to look like a party - like a Woodstock. And that's not what it was intended to be," she said. "It makes too light of the problems we have to address."

Indeed, the atmosphere at the capsite was more like that of a summner music festival than an angry protest. Demonstrators lolled about in the warm sun or played their guitars under the scrub pines.

Although 200 police from three states patrolled the area, New Hampshire Attorney General Thomas Rath told a press conference "it appears things should go as good as the weather."

The demonstration was, pure and simple, a media event. When Spock, the baby doctor and former antiwar activist, arrived and looked around, he said, "There's more press than demonstrators here."

One radio reporter conducted a lengthy interview with a television cameraman about 9:30 a.m. Demonstrators by the score were still marching into the site at 4 p.m.

Lynn Lazaroff, 19, from Pittsburgh, joined the Solar Rollers from Amherst, Mass., who bicycled 120 miles to get here. They stopped in town squares along the way to pass out muffins baked in a solar oven.

"The U.S. government has deceived us into thinking that nuclear power is clean and cheap and will provide everyone with jobs," Lazaroff said. "Well, that's a lie. The only way to turn that around is to talk to people on their porches and let them know there is more to energy than turning on a light switch."