Around 30 Greenpeace activists were arrested on Monday after breaking into an EDF nuclear power plant in southern France, saying they wanted to expose security flaws and demanding its closure.
The activists, dressed in red, said they reached the walls of two reactors at the Tricastin plant, one of France's oldest. EDF denied they had got into 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 who entered the plant at dawn 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."
All 30 protesters were arrested by 7 a.m. ET after several hours spent rounding up activists clinging to metal structures and ladders, EDF said.
Plant not on safety list
France's nuclear safety agency ASN 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.
Hollande sacked his energy and environment minister for publicly criticizing cuts to her budget earlier this month.
The president has pledged to cut the share of nuclear energy in the country's electricity mix to 50 per cent from 75 per cent 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 honour 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.