Swedish police arrested some 50 Greenpeace activists yesterday who broke into a nuclear plant demanding that parliament reject the building of new reactors, the environmental group and police said.

"Around 50 activists were arrested" at the Forsmark nuclear power plant near Uppsala, north of Stockholm, local police spokesman Christer Nordstroem said, adding he did not know the exact number of arrests.

"Now we think there are no activists left" on the premises, he added.

He later told Swedish news agency TT that some of the activists had been released after being identified, and that 11 were taken into custody for aggravated trespassing.

Swedish, German, Norwegian, French and Polish citizens were among those arrested, said another police spokesman, Olle Rehn.

Greenpeace put out a statement earlier yesterday saying around 30 people would enter the Forsmark facility to conduct a peaceful protest against an expected decision to expand nuclear power production in Sweden.

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

"The activists demand that Swedish members of parliament vote 'no' to new nuclear power on June 17 and instead bet on renewable energy," Greenpeace said, adding its protesters were ready to stay put until the day of the vote.

On its website, Swedish public radio showed activists dressed as white wind turbines and yellow sunshines - allusions to wind and solar power - with one activist preparing to go over the plant's fence using a ladder truck.

Marie-Louise Marsh, a security guard at the plant, told TT that she saw a fire truck, reportedly with French plates, back up to the fence, with three activists then hopping over the fence using ropes.

The power plant moved swiftly to increase security.

"We have heightened our preparedness level and secured all vital areas," Forsmark chief Stegan Persson said in a statement.

Prime Minister Fredrik Reinfeldt voiced concern, telling TT: "Security work is fundamental and decisive for the Swedish people's confidence in Swedish nuclear power production."

The Forsmark plant is owned by the Forsmark power group, whose largest owner is Swedish state-owned power giant Vattenfall.