On an August day last summer, 1,000 people gathered near the marshes of this picturesque seacoast town. There were speakers, protest songs and guerrilla theater, and when it was over the police had arrested 180 persons.

The civil rights movement is be-calmed. The Vietnam war is over. But the demonstrators, a little older now, their ranks swelled with new converts, are still around. Many of them have joined the growing antinuclear movement.

The movement, a coast-to-coast grass-roots network, organized anti-nuclear referendums in six states last year. All six were defeated, but the battle goes on, and, at the moment, the front line may be here.

"It's like war," says Guy Chickester, 42, a local carpenter who re-mortgaged his house and is spending full timefighting the Seabrook nuclear plant.

Chichester and 32 New England activists formed the Clamshell Alliance last year which organized the August demonstration. More than 5,000 people in anti-nuclear groups around the country are "committed" to come to Seabrook April 30 for a permanent occupation of the site, Chichester said.

Taking their cue from the successful 1974 occupation which stopped a proposed West German nuclear plant, Chichester said, "The Clams follow a philosophy of non-violence. We're willing to take roughing up and go to jail."

Foremost in their minds is the fear of radiation. "We're committing suicide," Chichester said. "The potential destructiveness of nuclear plants ranges from cancer-causing low-level radiation to the possibility of major meltdown to the possibility of major meltdown catastrophes [releasing radiation] to the creation of deadly plutonium which must be stored for 250,000 years."

Such fears are discounted by teh industry and fiercely argued among scientists, but they are an article of faith for groups like Clamshell Alliance and its more conservative cousins, Seacoast Anti-Pollution League and New England Coalition on Nuclear Pollution.

Attorneys for the latter groups, including Anthony Roisman, a Washington environmental lawyer, have led the Seabrook opposition in the hearing rooms of the regulatory agencies and in court.

Dorothy A. Anderson, of West Newbury, Mass., 35, a self-described "part-time activist, part-time homemaker and part-time organic gardener," is president of the 8-year-old Seacoast League.

"I see nuclear energy as the destruction of the planet, the destruction of mankind," she said. "It is particularly a women's issue because we will have to bear the deformed children due to an accumulation of radiation.

"Nuclear plants emit some radiation all the time. The government talks about permissible doses, but the permissible doses get lower all the time. By the time we get enough evidence, it will be too late. Cancer takes 20 years to develop."

Chichester and Anderson say Public Service Co. of New Hampshire, which is building the Seabrook plant, has vastly overestimated the region's future electricity demand. They point out New Hampshire has a 40 per cent reserve capacity - a fact the company does not dispute, but contends will be used up with rapid growth.

Conservation, solar energy and wood can satisfy future energy needs, they say. Also, they add, smaller, abandoned hydroelectric facilities could be reactivated - a possibility the company regards as impractical.

"Nuclear generation is the most capital intensive and labor regressive of all industries," Chichester said."With $2 billion - the cost of Seabrook - every New Hampshire household could be fitted with a solar space and water heating system. Thousand of people would be used to do the work."

The Clams and their friend think they're winning. "Historical forces are moving fast," Anderson said. "The country is rapidly turning away from nuclear power. Seabrook may be one of the last plants."

The statistic appear to support them. In the last three years, utilities have cut back drastically on new orders for reactors. In the last two years, 145 planned nuclear plants we're delayed for mostly economic reasons.