STOCKHOLM (Reuters) - Police in Sweden arrested dozens of Greenpeace activists on Monday after they broke into the Forsmark nuclear power plant ahead of a planned vote this week on whether to replace the country's existing reactors.

The activists entered Forsmark some 115 kilometres north of Stockholm early on Monday and several gained access to a building rooftop, police said. The protestors did not enter any of the operating areas.

"We got the alarm around 8 a.m. (0600 GMT) this morning. A few Greenpeace activists got over the fence and some even came by an inflatable boat from a nearby river," Christer Nordstrom, spokesman for the Uppsala police, told Reuters.

In a video posted on the website of Greenpeace Sweden, activists dressed in bright yellow 'sun' costumes were shown waving signs demanding that politicians strike down a proposed law on Thursday that would allow the replacement of Sweden's existing nuclear reactors.

Nordstrom said some 50 activists were being held on charges of breaking and entering and canine police units were checking whether any of the protestors remained inside the plant. The activists posed no danger to the plant's operation and none resisted arrest, he said.

Sweden is heavily reliant on nuclear power and needs to refurbish plants, many of which were built in the late 1970s. The proposal to allow new reactors has met with opposition from several parties, including the Greens.

Operated by state-owned power group Vattenfall, Forsmark was connected to the Swedish grid in 1980 and produces about one seventh of Sweden's electricity output, according to the company's website.

"We take this incident very seriously," Vattenfall said in a statement, adding it had declared a state of heightened security around the plant.