Police have arrested 34 Greenpeace activists who forced their way into a nuclear power plant in eastern France, the company that runs the plant, EDF, said.

The activists hung anti-nuclear banners from the Fessenheim plant, France's oldest in operation, but the country's nuclear safety authority said they did not enter into its buildings and its security was not compromised.

The protesters used a truck to force their way into the site early in the morning on Tuesday, according to activists outside.

Police gendarmes subsequently surrounded and entered the plant.

"Gendarmes have 56 activists under control and 34 have been arrested," an EDF spokesman said. "There has been no impact on the security of the plant, which continues to function normally."

President Francois Hollande has promised to close Fessenheim by 2016 and cut France's reliance on nuclear energy to 50 percent of its electricity mix from 75 percent now.

Greenpeace wants Fessenheim's two 900-megawatt reactors, which have been in operation since 1977, to be shut immediately.

In a statement on Tuesday Greenpeace France said the activists had come from 14 countries across Europe "to denounce the risk to Europe from France's nuclear power," and to promote a transition to other energy sources.

"The Fessenheim plant is a symbol," Greenpeace activist Cyrille Cormier said. "Its planned closure must be the beginning of a series of plant closures in Europe to limit the accidental and financial risks linked to ageing [plants] and to start the energy transition."

The activists hung a banner from the roof of the plant that said "stop risking Europe" and called on Hollande and German Chancellor Angela Merkel to commit at an EU summit on Thursday to generating energy from alternative sources.

Greenpeace activists have a history of breaking into nuclear plants in France and about 30 were arrested last July after entering EDF's Tricastin plant in southern France.