About two dozen activists from the international environmental campaign group Greenpeace have climbed fences to break into a nuclear power plant in southern France.
The activists entered the state-run Electricity of France’s (EDF) Tricastin plant about 200km north of Marseille at around 03:00 GMT on Monday and projected visuals on the building, Greenpeace and police said.
The images read, "Tricastin, nuclear accident, ready to pay the price?" and "Francois Hollande: president of a catastrophe?" in reference to the French president, Isabelle Philippe, a Greenpeace spokeswoman, said.
Twelve of the activists were arrested more than two hours later, according to EDF, which runs the country's atomic power plants.
A spokesman for the interior ministry said the activists "were not able to access the plant's sensitive areas, notably command rooms".
"It's a media stunt that poses no security danger," said spokesman Pierre-Henry Brandet.
Brandet said that the activists, who divided into three groups upon entering the plant, "were immediately detected" and that about 50 gendarmes were deployed to arrest the remaining trespassers.
Speaking at a news conference French President Francois Hollande said France is very much attached to nuclear safety, and that everything was being done "to assure us that this nuclear safety is absolutely respected."
Members of the environmental anti-nuclear group have staged several break-ins at French nuclear plants in recent years in an effort to highlight the "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 broke into the Nogent-sur-Seine plant 95km 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. Hollande pledged to cut the share of nuclear energy in the country's electricity mix to 50 percent by 2025.
He also wants to close the country's oldest plant at Fessenheim, near the German border, by 2017.