Paris (CNN) -- French police arrested dozens of Greenpeace activists Tuesday who had forced their way into a nuclear power plant.
During the early morning break-in, the activists hung anti-nuclear banners from the Fessenheim plant, France's oldest in operation and a flashpoint for anti-nuclear campaigners who say it is unsafe and should have been closed long ago.
"Today, militants of various nationalities, coming from all over Europe, protested and occupied Fessenheim, the oldest French nuclear center," Greenpeace said on its website.
EDF, which operates the plant in eastern France, said 56 people had been detained.
Local authorities said police remained on site as a precautionary measure.
"No activist entered inside the buildings. These events had no impact on the safety of facilities, which are operating normally," local authorities said in a written statement.
Greenpeace wants Fessenheim, which has been in operation since 1977, to be closed immediately.
The activists hung a banner from the roof of the plant that said "stop risking Europe" and called on French President Francois Hollande and German Chancellor Angela Merkel to commit to generating energy from alternative sources.
Hollande has pledged to close the plant by the end of 2016.
CNN's Elaine Ly, Sandrine Amiel, Boriana Milanova and Marie-Louise Gumuchian contributed to this report