SEABROOK, N.H.--Anti-nuclear demonstrators swarmed over and crawled under the outer perimeter fence at the Seabrook nuclear power plant yesterday, and at least 600 were arrested without resistance.

The number of arrests was the second highest of the many demonstrations that have been held at the plant, which has been a focal point for the anti-nuclear movement since construction began 13 years ago. Previously, more than 2,500 arrests had been made, including 1,414 in 1977.

The weekend protests also were the largest in 11 years and coincided with preparations at the plant for low power testing, expected to begin late this week.

The demonstrators, including children and a woman in a wheelchair, climbed or were helped over the seven-foot high chain-link barrier. Some carried infants.

Even as arrests were being made about a half-mile inside the fence, another wave of demonstrators scaled the fence using home-made ladders.

Hundreds of other protesters remained outside the fence, and about 50 more were arrested early in the evening when they scaled the fence. Still others planned to enter the grounds later.

There were no reports of injuries.

Dianne Dunfey, one spokesperson for the protesters, said demonstrators planned to be back at the plant this morning to block the gates when workers reported for work.

State Attorney General John Arnold said as many as 650 protesters were arrested and taken in school buses to the Seabrook police station.

Dispatcher Blanche McCallion of Seabrook police said those arrested were charged with criminal trespass and were being released with a summons.

"We're sending a message out across the country that the antinuclear resistance is rededicating opposition to Seabrook Station and we're calling upon the movement across the country to recognize the dangers that are represented at every nuclear power plant," Paul Gunter of the organizing Clamshell Alliance said, leaning out of one bus after his arrest.

Plant officials inside said 51 people were arrested at a railroad entry point to the site, where demonstrators reenacted a 1976 protest and then climbed onto the plant property. Another 81 were arrested inside the north gate and hundreds more inside the south gate, where they sat down after encountering a police line.

Among those arrested was Massachusetts state Rep. Nicholas Costello (D-Amesbury) and former congressional candidate Burt Cohen of New Hampshire.

The demonstrators walked without protest to waiting school buses.

The protesters, who were singing and chanting "No Nukes," were warned over bullhorns that they were on private property and subject to arrest, but they linked hands and refused to leave. When confronted by police, most sat down.