SEABROOK, N.H.--Anti-nuclear protesters failed yesterday afternoon to prevent a reactor pressure vessel from passing through the south gate of the Seabrook nuclear power plant.

Chanting "Remember Three Mile Island--can we forget Seabrook?," about 250 activists assembled at the gate to show their continued opposition to the construction of the plant. Individual protesters attempted to sit in the path of the 450-ton vessel, but police quickly removed them.

The demonstration was "peaceful..no arrests, no one badly hurt," Paul Cronin, Seabrook police chief, said yesterday.

But Nate Thayer, a spokesman for the Boston Clamshell Alliance, an anti-nuclear group, charged yesterday that "the police were excessively violent with absolutely no provocation," He added that a woman had been hospitalized with a concussion suffered during a skirmish with the police.

The reactor vessel, in which the fission takes place, is "the exact same design as the vessel that came within 40 minutes of melting down at Three Mile Island," Thayer said.

But John Cavanough, manager of the Seabrook Education Center, said yesterday that human error, not equipment failure, caused the leakage of radioactive gas at Three Mile Island. He added that Seabrook's computer-simulated operator training program will virtually eliminate the risk of human error.

Leaving the docking facilities at Hampton Harbor at 9.a.m., 96-wheeled truck transported the $3 million cylindrical vessel to Seabrook over a seven-mile stretch of road, which Gov. Hugh Gallen (D.-N.H.) had ordered blockaded. About 150 New Hampshire state troopers and 21 Massachusetts state troopers escorted the truck "for protection purposes," Jennifer Murphy, Gallen's press secretary said yesterday.

Gallen requested the Massachusetts troopers because a two-mile stretch of the road crossed the Massachusetts-New Hampshire border, Roy Lions, assistant press secretary to Gov. Edward L. King (D.-Mass.), said yesterday.

"The Public Service Company, which owns the Seabrook plant, will file with the Nuclear Regulatory Commission for an operating license this year and hopes to begin operating its first reactor in 1983, Norman Cullerot, manager of nuclear information, said yesterday