The police, who sealed off the area after the attack, found several rocket launchers and a carrying case bearing inscriptions in the Cyrillic alphabet. They said rockets and rocket launchers of this type, RPG-7's dating from 1963, could be bought in almost any illegal arms market in Europe for about $500 each.

A group calling itself the Pacifist and Ecologist Committee said in a telephone call to Agence France-Presse that it staged the attack. A regional ecological association denied responsibility but said despair over Government plans to complete the plant could have provoked individual militants into mounting the attack.

Major ecological and conservation groups in France denounced the attack but said they still opposed France's ambitious nuclear program, one of the largest in the world. A spokesman for the Friends of the Earth said, ''We are not surprised that some people have turned in that direction after the deception worked on us by the Socialist Government.'' Government's Record

''The Socialists have made pledges on Superphoenix and the rest of the nuclear program and they've broken them,'' the spokesman, Remi Parmentier, said. ''It's a very serious situation and the Government must rethink its position.''

One of President Francois Mitterrand's first official acts after taking office in May was to halt work on nine nuclear plants then being built. It was a freeze, his aides said, to enable the new Government to study the country's nuclear energy program. Mr. Mitterrand canceled plans for a nuclear plant at Plogoff on the Brittany coast, where there had been frequent clashes between residents and the police.