Over 20 Greenpeace activists climbed fences to break into an EDF nuclear power plant in southern France this morning and demanded its closure, the environmental campaign group said.

The activists, dressed in red, broke into the Tricastin plant at dusk and unfurled a yellow and black banner on the wall saying above a picture of President Francois Hollande: "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.

A spokeswoman for EDF denied the activists had reached two of the plant's reactors and said that by 0630 GMT, 17 of them had been arrested for unauthorised access. Others clung onto metal structures and ladders, she said.

Hollande pledged to cut the share of nuclear energy in the country's electricity mix to 50 percent from 75 percent by 2025. He also said he wanted 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 over 30 years ago.