Breaking into a French nuclear power plant before dawn Monday, Greenpeace activists say they have exposed the vulnerability of the site as they unfurled a banner on one of its domes reading: Safe Nuclear Power Doesn't Exist.

Nine activists bypassed security at the Nogent-sur-Seine plant, about 59 miles southeast of Paris, and scaled a domed containment building above a nuclear reactor before being arrested. It was one of at least four near-simultaneous attempts to invade nuclear sites across France, The Associated Press reported.

The aim is to show the vulnerability of French nuclear installations, and how easy it is to get to the heart of the reactor, said Sophia Majnoni, a Greenpeace nuclear campaigner.

In about 15 minutes, the activists reached the heart of the plant, where the nuclear core and nuclear fuel are.

French authorities confirmed the intrusion and said a thorough search was being conducted of all installations.

President Nicolas Sarkozy condemned the rather irresponsible risks to lives, while the French energy firm, Electricite de France, or EDF, tried to downplay the incident, saying it had been aware of the intrusion all along.

We saw immediately that they were not armed. As soon as these people entered they were detected and followed, and when you are dealing with people with peaceful intention, you must intervene calmly, Dominique Miniere, the head of EDF's nuclear program, said in a press conference.

This incident must not be exaggerated, he said, adding that the activists were unable to enter buildings or other highly protected areas.

The intrusion further stoked concerns of the vulnerability of France's nuclear facilities to terrorists or any other would-be invaders. France, a huge supporter of nuclear power, gets about three-quarters of its electricity from it, more than any other nation. France operates 58 reactors and has been a leading international proponent of atomic energy.

After the Fukushima disaster in Japan, France's reliance on nuclear power has been increasingly called into question. Last month, Sarkozy said it would be madness for France to reduce its reliance on nuclear power; he has promised full transparency about the safety of nuclear facilities in France.

The main opposition Socialist party's nominee in France's next presidential election, Francois Hollande, has pledged to shut down more than 20 reactors.