A French court Thursday slapped two-month suspended prison sentences on 55 Greenpeace activists who launched an audacious break-in at France's oldest nuclear power plant to highlight weaknesses at atomic installations.

Only three of the 55 defendants turned up in court in the eastern French city of Colmar to face trial over the March 18 protest at the Fessenheim power plant near the border with Germany and Switzerland.

The activists -- who were all convicted of trespassing and causing wilful damage -- included 21 Germans, seven Italians and people from several other nationalities including France, Turkey, Austria, Hungary, Australia and Israel.

Colmar prosecutor Bernard Lebeau, who had sought three to four month suspended sentences, said although debate on nuclear energy was "perfectly legitimate", their actions were completely "illegal."

"It was my duty to do this and I did it," said Eddy Varin, a French 41-year-old civil servant who was among the protesters. "It was legitimate."

"To create public awareness, one sadly sometimes has to do media stunts to put pressure on elected representatives," said Jean-Michel Vourgere, a computer specialist from Paris.

- 'Francois, you promised' -

The three defendants who came to court, including 73-year-old German national Peter Wendt, unfurled a banner outside which read: "We are disobeying to secure a better future."

As the trial got underway, a handful of supporters also gathered outside with a sign proclaiming: "Greenpeace activists are the heroes of our time."

They also held up cartoons of President Francois Hollande and Environment Minister Segolene Royal with a caption: "Shut Fessenheim."

The March dawn protest took place at the plant run by state-run power firm EDF.

About 20 activists had managed to climb up onto the dome of one of the reactors but EDF said the safety of the plant had not been compromised, and the protest ended hours later.

France, the world's most nuclear-dependent country, operates 58 reactors and has been a leading international cheerleader for atomic energy.

But in a deal with the Greens before the 2012 parliamentary and presidential elections, Hollande's Socialist Party promised to cut reliance on nuclear energy from more than 75 percent to 50 percent by shutting 24 reactors by 2025.

Hollande has pledged to close Fessenheim, which was commissioned in 1977, by the end of 2016.

"Francois, you promised," read one Greenpeace banner on Thursday, in reference to this pledge.

The plant, located on the banks of the Rhine, is considered vulnerable to seismic activity and flooding.

The protest stunt came ahead of a meeting by European leaders to discuss the future of the continent's energy policy.

Greenpeace wants Hollande and German Chancellor Angela Merkel to push Europe towards cleaner energy, complaining that France relies too much on nuclear power and Germany on coal for electricity supplies.

Hollande has repeatedly pledged to develop renewable energy and vowed to improve the energy efficiency of one million badly insulated homes.

France plans to reach the EU's 10-percent renewable energy target by boosting the use of second-generation biofuels, which are made from crop residues, waste, algae or woody material.