About 20 Greenpeace activists took part in the operation. Six of them chained themselves to the upper part of the main entrance gate to the plant in northern Garona, which dates back to 1971.

The others, attached by the neck to the lower part of the gate, unfurled a banner urging: "Zapatero, close the nuclear plants now!"

Socialist Prime Minister Jose Luis Rodriguez Zapatero promised when he took office in April to gradually phase out nuclear power and reform Spain's energy policy. He promised to develop alternative sources of energy to make the sector more environmentally-friendly and reduce emissions of the greenhouse gases that cause climate change.

"The Garona plant is the worst in Spain and one of the most dangerous in Europe. It must be immediately closed for good because it is not safe," Greenpeace campaigner Carlos Bravo told AFP.