Dozens of Greenpeace activists sneaked into France's oldest nuclear power plant Tuesday to highlight alleged security weaknesses at atomic facilities, prompting the government to pledge to reinforce protection of the sensitive sites.

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, which the environmental group said aimed "to denounce the risk of French nuclear power for the whole of Europe", prompted Ecology Minister Philippe Martin to take immediate action.

In a statement, he said he would "ask operators to reinforce the physical protection of the most sensitive zones in their nuclear facilities", by asking them to put up protective devices such as high-voltage barriers.

The break-in ended late in the afternoon, and state-run power firm EDF, which runs the plant, said it had had no impact "on the safety of facilities, which functioned normally."

Police detained 56 activists, around 20 of whom managed to get onto 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 -- a promise re-affirmed Tuesday by Martin.

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 EU's 10 percent renewable energy target by boosting the use of second-generation biofuels, which are made from crop residues, waste, algae or woody material.