About 1,000 demonstrators gathered at a parking lot one mile from the plant Sunday morning and began marching toward the seaside plant shortly before 1 p.m. Another 350 to 400 protestors already were blocking one of the gates.
Before marching on the plant, members of the organizing Clamshell Alliance formed a large circle to honor the Chinese killed this weekend when troops crushed the student protest in Beijing.
State Police Capt. Sheldon Sullivan said 100 state troopers and 54 officers from Seabrook and the neighboring communities of Hampton, Portsmouth and Exeter were deployed in the demonstration area.
Forty members of a New Hampshire Air National Guard security unit were standing by at Pease Air Force Base about 15 miles away, at the request of Gov. Judd Gregg.
Seabrook opponents maintain that the plant is not safe and that the congested beach tourist area around it could not be evacuated safely in case of an accident.
Plant spokesman Ron Sher said the demonstrators are a "vocal minority."
"The majority of people in New Hampshire, Massachusetts and the rest of New England recognize that nuclear power is a viable energy alternative and Seabrook...is ready to produce electricity and it can make an important contribution," he said.
The plant has won a low-power testing license, but cannot run commercially until its emergency plans for surrounding communities in New Hampshire and Massachusetts are approved.
Hearings have been completed on New Hampshire's plans, and a ruling is expected in the fall from a government panel. Massachusetts has refused to submit plans and hearings continue on plans the utility has submitted for the Massachusetts communites.
Since receiving its low-power license last month, the long-delayed $6 billion plant has been warming its reactor in preparation for a its first nuclear reaction.
More than 4,000 protesters rallied near the plant Saturday. Bands played and protesters sang and listened to speakers denounce the plant.
One opponent, Stephen Comley, hired an airplane pulling an antinuclear banner to fly by President Bush's vacation residents in Kennebunkport, about 40 miles up the Maine coast. Bush was spending the weekend there.