Before dawn on Monday, Greenpeace activists slipped past security and gained access to the nuclear power plant in Nogent-sur-Seine, some 95 kilometers south-east of Paris.

Greenpeace claimed that some of the activists managed to climb on the roof and unfurl a banner.

EDF, the state electricity company, which runs all of the country's 58 nuclear reactors, confirmed there had been a security breach. However, they said the intruders had been "immediately detected" and that the incident did not pose a safety threat.

Seven activists were arrested by police.

Activists also tried to break into at least three other plants elsewhere in the country. EDF said there were "no traces of intrusion" in other stations.

"We are proceeding with an extensive search of all nuclear plants and installations," spokesman for the interior ministry Pierre-Henry Brandet told the news agency AFP.

Industry Minister Eric Besson expressed surprise at the incidents and said an investigation was underway. "We will have to take measures to ensure it doesn't happen again," he told France Info radio.

Around 75 percent of electricity in France is generated by nuclear power.

Since the Fukushima nuclear disaster in Japan earlier this year, the government has come under pressure to reduce the country's reliance on nuclear power.

Author: Joanna Impey (AFP, AP, dpa)
Editor: Andreas Illmer