PARIS - Around 30 Greenpeace activists climbed fences to break into a nuclear power plant in southern France at dawn on Monday, saying they wanted to expose security flaws and demand its closure.
The activists, dressed in red, said they reached the walls of two reactors at the Tricastin plant, one of France's oldest. Operator EDF denied they had gained access to any "sensitive areas" and said production was not affected.
Interior Minister Manuel Valls called for an investigation into the intrusion which raised questions about the security of France's 19 nuclear plants and 58 reactors.
The protesters unfurled a yellow and black banner on a wall above a picture of President Francois Hollande, marked with the words: "Tricastin, nuclear accident - President of the catastrophe?"
"With this action, Greenpeace is asking Francois Hollande to close the Tricastin plant, which is among the five most dangerous in France," Yannick Rousselet, in charge of nuclear issues for Greenpeace France, said in a statement.
"If being physically able to touch the reactors is not being in a sensitive place, I don't know what is," Rousselet told Reuters. "People with bad intentions could have posed a threat to the reactor's safety."
Most of Monday's protesters were arrested by 8:30 a.m. local time (2:30 p.m. ET) but around eight of them were still clinging to metal structures and ladders, EDF said.
"So far, the incident did not have any impact on the facility's safety," France's nuclear safety agency ASN said.
The agency did not include Tricastin in a list released in April of six nuclear plants with the lowest safety performance in 2012.
The action echoed tensions between the Socialist government and ecologists, who accuse Hollande of not doing enough to reduce France's reliance on nuclear power and increase the use of renewable sources of energy.
The president has pledged to cut the share of nuclear energy in the country's electricity mix to 50 percent from 75 percent by 2025. He has also said he wants to close the country's oldest plant at Fessenheim, near the German border, by 2017.
Greenpeace said to honor his promise, Hollande would have to close at least 10 reactors by 2017 and 20 by 2020. The campaign group said this ought to include Tricastin, which was built more than 30 years ago.
The dawn raid came less than a week after six female Greenpeace activists climbed London's Shard, the tallest building in Western Europe, in protest over plans by oil producer Royal Dutch Shell to carry out drilling in the Arctic circle.