Dozens of Greenpeace activists broke into the Tricastin nuclear power plant in southern France on Monday, to call for its closure and pressure President Francois Hollande to put more resources into renewable energy.

The activists slipped into the four-reactor plant near the city of Avignon before dawn and hung two giant banners bearing Hollande’s image and the slogan: “Tricastin nuclear accident. President of catastrophe?” near the reactors no. 1 and no. 3.

They also projected the slogan “Tricastin nuclear accident” and a forked line representing a fissure in green luminous letters on the wall of a building.

The interior ministry said 21 activists had been detained. None had been able to access sensitive areas “namely the control rooms,” the ministry said.

Greenpeace said the ease with which the activists reached the reactors demonstrated the vulnerability of the facility from a security perspective.

The action was aimed at pressuring Mr. Hollande to close the facility, which Greenpeace calls “one of the five most dangerous facilities in France.” The 33-year-old plant is the third-oldest in France’s nuclear park. Multiple cracks on the reactor vessel no. 1 and its location in an area at risk of flooding and tremors make it a safety hazard, according to Greenpeace.

The French nuclear safety authority (ASN) says the fissures pose no safety threat.

France gets about three-quarters of its electricity from 58 nuclear reactors. Mr. Hollande has promised to reduce the share of nuclear in the energy mix to 50 per cent by 2025.

So far, however, Mr. Hollande has announced plans to close only one plant, at Fessenheim on the border with Germany. Greenpeace says more plants need to be closed if the President is to meet his target.