Almost 2,000 peaceful protesters have pitched camp tonight on the grounds of a controversial nuclear electric generating plant under construction here in what is being billed as the nation's largest demonstration yet against nuclear power.

The members of the Clamshell Alliance, a loose-knit coalition of nuclear power opponents, were allowed by state officials to camp in a dirt parking lot on the grounds outside the fenced-in construction area for the weekend.

Mass arrests that had been threatened all week by Gov. Meldrim Thomson and officials of the Public Service Co. of New Hampshire were thus averted.

State officials did not say if the permission to camp would be extended beyond the weekend. Protest organizers planned to meet Sunday to decide how long they will attempt to stay.

Camped inside the construction area are more than 300 police with dogs and National Guard logistical support troops.

The protesters, remarkably organized by the standards of the demonstration of the 1960s and almost all trained in nonviolence over the past several months, convered on the site around 3 p.m. in military-like columns coming from four directions, including about 300 who had been ferried by boat and then marched up from a beachhead they had established.

"No Nukes" said the sign carried at the head of the main column by Bud Ross, 33, a rehabilitation counselor who, like many of the protesters here, is a New England veteran of the civil rights and antiwar protests of the '60s but who has been silent since.

"Spectacular," he said to a reporter as the column marched through barriers and headed down the long access road to the site.

About 10 small support demonstrations were being held at nuclear power plants across the country, said Clamshell organizers. The aim, they said, is to carry the fight against nuclear power from the courtrooms to the streets in hopes of stopping it by nonviolent means.

The planned Seabrook plant is at the moment little more than holes in the ground, surrounded by several pre-fab warehouses and construction equipment. One month ago, the Nuclear Regulatory Commission limited construction pending a decision by the Environmental Protection Agency on the plant's controversial cooling system.

The "once through" cooling system is a unique one designed to be more efficient, but which also discharges water into the ocean at high temperatures, which environmentalists claim destroys the ecological balance of the coastal waters.

Clamshell and other groups claim also that the construction is destroying the ecologically rich saltwater marsh in which the plant is being built.

Almost 180 persons were arrested last fall here in a much smaller demonstration. A county judge afterward issued an injunction against trespassing on the company's property, but lifted that injunction Friday on the grounds now occupied by the protesters.

The protest was elaborately organized over many months by Clamshell, which is divided into cell-like "affinity groups" of about a dozen people each. Each group trained and marched together and is camping together.

Organizers said that in anticipation of this protest some 1,800 persons throughout New England went through a training program in non-violent disobedience set up with the assistance of the Quaker-affiliated American Friends Service Committee.

Clamshell marshals strictly enforced that only trained persons were allowed to march and that drugs, alcohol, weapons and thelike were prohibited.