Dozens of Greenpeace activists sneaked into France's oldest nuclear power plant Tuesday in the latest break-in by the environmental group to highlight alleged security weaknesses at atomic facilities. The 60-odd activists broke into the Fessenheim plant in eastern France near the border with Germany and Switzerland and hung a banner reading "Stop risking Europe" on the side of one of its reactors. The action was "to denounce the risk of French nuclear power for the whole of Europe," the group said in a statement. A spokesman for state-run power firm EDF, which runs the plant, said it was "functioning normally" despite the break-in that ended late afternoon. Police had detained 56 activists, he said. About 20 of them managed to get on to the dome of one of the reactors as a police helicopter hovered above. Later another group of Greenpeace activists put up a giant banner next to the nearby Rhine canal, which read "Future is renewable, stop nuclear." France, the world's most nuclear-dependent country, operates 58 reactors and has been a leading international proponent of atomic energy. But in a deal with the Greens before the 2012 parliamentary and presidential elections, President Francois Hollande's Socialist party promised to cut reliance on nuclear energy from more than 75 percent to 50 percent by shutting 24 reactors by 2025. Hollande has pledged to close Fessenheim, which was commissioned in 1977, by the end of 2016. The ruling Socialists' Green partners welcomed Tuesday's protest, which they said "shed light on the fragility of our nuclear installations." The plant, located on the banks of the Rhine, is considered vulnerable to seismic activity and flooding. But Thierry Rosso, the man in charge of the Fessenheim plant, said the protest "shows that it is not possible to access the most important zone" where the sensitive nuclear installations are located. The protest stunt comes ahead of a meeting by European leaders to discuss the future of the continent's energy policy. Greenpeace wants Hollande and German Chancellor Angela Merkel to push Europe towards cleaner energy, complaining that France relies too much on nuclear power and Germany on coal for electricity supplies. Hollande has repeatedly pledged to develop renewable energy and vowed to improve the energy efficiency of one million homes that are badly insulated. France plans to reach the 10-percent European Union target of renewable energy in fuel by boosting the use of second-generation biofuels, which are made from crop residues, waste, algae or woody material.