SEABROOK, N.H.--Several thousand nuclear protesters will attempt an illegal takeover at dawn today of the partially constructed nuclear power plant here.

Last night, under a heavy mist cover, protesters gathered at more than 30 camp sites near the plant, while several hundred National Guardsmen and police from five New England states prepared to confront them.

State officials, who say they will do what is necessary to protect the plant, have said they will arrest anyone who trespasses on the site.

"If there's any violence tomorrow, it will be brought on by the authorities," Edgar Bittone of the Coalition for Direct Action (CDA), the Clamshell Alliance offshoot that is organizing the protest, said yesterday.

After cutting down fences to enter the 715-acre plant site, protesters will not commit any acts of violence or destroy any property, Bittone said.

The occupation attempt, the first of its kind in the United States, is aimed at preventing construction at the plant by establishing a community there.

Comin' Home

Estimates of the number of protesters planning to take part in today's occupation range from 2000 to 10,000. Busloads of demonstrators, including a large contingent from Cambridge, began arriving in the early evening.

Organizers say the occupation attempt will be "open-ended," continuing until they are satisfied that construction of the plant will be halted. "We'll stay there one week, two weeks--as long as it takes," one protester said last night.

Plans call for protesters to gather at about 7:30 a.m. and trudge through surrounding marshlands to the plant's outer fence by 10 a.m.

If they avoid arrest, the protesters will head first for the building which will house the nuclear core of one of the two reactors under construction at Seabrook.

The two partially-constructed reactors--25 per cent and 5 per cent complete--are slated to begin operating in 1984 and 1989, respectively.

Estimated costs for the twin-reactors, which the Public Service Company of New Hamphire will operate, have risen from less than $900 million to $2.5 billion since construction began in 1976.

In April 1977, 1415 protesters were arrested in the largest previous mass occupation attempt in the anti-nuclear movement in the U.S.