* Activists target France's oldest plant before EU summit
* EDF says break-in has no impact on electricity output (Adds details, updates number of arrests, changes dateline)
By Patrick Genthon
FESSENHEIM, France, March 18 (Reuters) - French police arrested 57 Greenpeace activists who used a truck on Tuesday to ram their way into a nuclear power plant operated by EDF in eastern France, the organisation said.
The activists hung anti-nuclear banners from the Fessenheim plant, the oldest in operation in France.
Greenpeace activists have a history of breaking into nuclear plants in France and about 30 were arrested last July after entering EDF's Tricastin plant in southern France.
France's nuclear safety authority said that in the latest protest, the activists did not enter buildings and the Fessenheim plant's security was not compromised.
A CGT union official said all French nuclear plants were on maximum security alert following the incident.
An EDF spokesman said monitoring had been stepped up and precautionary measures taken.
The protesters used a truck to break through two security barriers early in the morning, a local government official told journalists. Police later surrounded and entered the plant.
"There has been no impact on the security of the plant, which continues to function normally," the EDF spokesman said.
After police wound down the protest inside the plant, more activists arrived on a canal outside on inflatable rafts and unfurled anti-nuclear banners.
President Francois Hollande has promised to close Fessenheim by 2016 and cut France's reliance on nuclear energy to 50 percent of its electricity mix from 75 percent now.
Greenpeace wants Fessenheim's two 900-megawatt reactors, which have been in operation since 1977, to be shut immediately.
"The Fessenheim plant is a symbol," Greenpeace activist Cyrille Cormier said. "Its planned closure must be the beginning of a series of plant closures in Europe to limit the accidental and financial risks linked to ageing (plants) and to start the energy transition."
The activists hung a banner from the roof of the plant that said "stop risking Europe". They called on Hollande and German Chancellor Angela Merkel to commit at an EU summit on Thursday to generating energy from alternative sources. (Reporting by Gilbert Reilhac, Patrick Genthon, Michel Rose and Marion Drouet; Writing by Leigh Thomas; Editing by John Stonestreet and Ruth Pitchford)