The activists were arrested on suspicion of trespass and breaching the 1990 Protection of Important Installations Act (Law 1990:217).

The activists climbed and fence and entered the plant on Monday morning demanding that parliament vote on June 17th against the building of new nuclear power facilities.

Police confirmed that 50 people have been arrested but that there may be more hiding on the premises and the area is being searched by police and several canine units have been despatched.

“I think that there are more than these people. We are searching the area now,” police spokesperson Olle Rehn.

The environmental group itself sent out a statement earlier saying around 30 would enter the facility to conduct a peaceful protest against nuclear power use.

“The activists demand that Swedish members of parliament vote ‘no’ to new nuclear power at a vote on June 17 and instead bet on renewable energy,” the group said, adding that the protesters were prepared to stay put until the day of the vote.

Sweden, which has 10 reactors at three power stations, announced last year it had reversed a decision to phase out nuclear power, and the parliament is set to vote Thursday on whether to expand the Scandinavian country’s nuclear facilities.

Greenpeace spokesman Ludvig Tillman told the TT news agency several activists had been stopped by guards, but that “many succeeded in getting in”.

The activists, who reportedly climbed over a fence to get in, were protesting an expected “decision that will have consequences for hundreds of years if you think of the waste nuclear power creates,” he said. “Replacing existing nuclear plants with new nuclear power would be a mistake of historical proportions,” he said in the Greenpeace statement.

Forsmark spokesperson Claes-Inge Andersson told TT that plant authorities were also following closely the protest, but said there was little worry the activists would pose a security risk.

“It is one thing to manage to climb over a fence, but it is not be possible for them to reach the vital parts” of the nuclear plant, he said, adding that the activists “are not here to damage the plant. They just want to protest a parliamentary decision.”