Several dozen Greenpeace campaigners snuck into a nuclear plant in southern France at dawn on Monday, in the latest such break-in by the environmental group.
The activists managed to enter the grounds of the Tricastin plant, some 200 kilometres (120 miles) north of Marseille, at around 5:00 am (0300 GMT), Greenpeace and police said.
They hung yellow banners reading "Tricastin: a nuclear accident" and "Francois Hollande: president of a catastrophe?" in reference to the French leader, according to Isabelle Philippe, a spokeswoman for the environmental anti-nuclear group.
"Greenpeace wants to point out all the security weaknesses in the production of nuclear energy," she said. "Tricastin is one of the most dangerous plants and one of five that should be closed quickly."
Nearly four hours after the initial break-in police had arrested 21 of the activists, including French, Italian, Romanian and Spanish nationals, according to the interior ministry.
Another dozen still inside the plant had been "located," said ministry spokesman Pierre-Henry Brandet.
Police said that arresting the activists was taking some time as some of them had chained themselves to structures inside the plant.
The activists "were not able to access the plant's sensitive areas," like command rooms, Brandet said.
"It's a media stunt that poses no security danger," he said.
He insisted that the environmentalists, who divided into three groups upon entering the plant, "were immediately detected" and that around 50 gendarmes were deployed to arrest the remaining trespassers.
The EDF energy giant that runs France's atomic power plants had also said that the activists did not manage to reach any sensitive areas within the site.
France's interior and energy ministers called for an investigation into the incident.
Greenpeace has staged several break-ins at French nuclear plants in recent years in an effort to highlight what they say are dangers of atomic power and to expose security problems at the power stations.
In May 2012, an activist with the group flew into the grounds of the Bugey plant in southeastern France using a hang glider in a stunt aimed at revealing alleged security flaws. He flew over the plant, threw a smoke bomb and landed inside before being arrested.
In December 2011, nine activists snuck into the Nogent-sur-Seine plant 95 kilometres southeast of Paris. Most were quickly arrested, but two managed to evade capture for nearly two hours.
France is heavily reliant on nuclear power, with its 58 nuclear reactors producing some 75 percent of the nation's electricity.
The Tricastin plant went online in 1980 and last year produced 24 billion kilowatt hours of electricity, which is the equivalent of consumption of around 3.5 million people, according to EDF.