Police on Monday arrested 29 Greenpeace activists who broke into a nuclear plant in southern France.

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The action was the latest break-in by the environmental group, aimed at highlighting alleged security weaknesses at atomic facilities.

The activists managed to enter the grounds of the Tricastin plant, around 200 kilometres north of Marseille at around dawn.

They hung yellow banners reading "Tricastin: a nuclear accident" and "Francois Hollande: president of a catastrophe?" in reference to the French president.

Before entering the facility, they also projected images inside the plant, including one showing a crack running along part of the structure.

"Greenpeace wants to point out all the security weaknesses in the production of nuclear energy," said Isabelle Philippe, a spokesperson for the group.

"Tricastin is one of the most dangerous plants and one of five that should be closed quickly", she said. "It was the easiest thing in the world for the activists to enter the plant, it took them 20 minutes to get from the entrance to the top of the structures."

The interior ministry said all the activists had been detained in a full sweep of the facility. It had taken several hours to arrest them all, after some had chained themselves to structures inside the plant.

Among those arrested were French, Italian, Romanian and Spanish nationals.

The government said in a statement that it was considering "stiffer penalties" for such actions.

The activists "were not able to access the plant's sensitive areas,” such as command rooms, interior ministry spokesman Pierre-Henry Brandet said, adding: "It's a media stunt that poses no security danger."

He declared that the environmentalists, who divided into three groups upon entering the plant, "were immediately detected".

EDF, the energy giant which runs France's atomic power plants, 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.

The head of the Green party faction in France's Senate, Jean-Vincent Placé, hailed Greenpeace's "civil action" for "alerting the French" to the potential dangers of nuclear power.

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.



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