Environmental group Greenpeace said Monday it had exposed the "vulnerability" of French nuclear sites after its activists broke into an atomic power station near Paris before being arrested.

French authorities admitted to security "lapses" after the incident and vowed a full investigation, while President Nicolas Sarkozy denounced the activists as "irresponsible".

The dawn raid saw nine activists sneak past security at the Nogent-sur-Seine plant, 95 kilometres (60 miles) southeast of Paris. Most were quickly arrested but two managed to evade capture at the plant for nearly two hours, authorities said.

"The aim is to show the vulnerability of French nuclear installations, and how easy it is to get to the heart of a reactor," said Sophia Majnoni, a Greenpeace nuclear campaigner.

"In about 15 minutes the activists reached the heart of the plant, where the nuclear core and nuclear fuel are," she told journalists.

French authorities confirmed the intrusion, saying the nine had been detained.

After initially insisting that "there are no traces of intrusion in EDF's other nuclear stations," late on Monday EDF said two Greenpeace activists had managed to enter the grounds of a reactor in the southern town of Cruas some 600 kilometres south of Paris.

It said the pair did not gain access to any restricted areas and had been handed over to authorities.

The French energy giant, which runs the nuclear plants that France relies on for 75 percent of its energy, sought to downplay Monday's incidents, insisting it had been aware of the intrusion at Nogent-sur-Seine from the start.

"We saw immediately that they were not armed ... As soon as these people entered they were detected and followed, and when you are dealing with people with peaceful intentions, you must intervene calmly," Dominique Miniere, the head of EDF's nuclear programme, told a press conference.

"This incident must not be exaggerated," he said, noting that at no point were the activists able to enter buildings or other "highly protected" areas.

Authorities insisted the incident had posed no risk.

"At no point was there a threat to the integrity of nuclear installations," interior ministry spokesman Pierre-Henry Brandet told AFP.

Authorities said attempts had also been made to break into nuclear power plants in Blayais in southwestern France and Chinon in the centre, as well as a nuclear research centre in Cadarache in the southeast.

Energy Minister Eric Besson's office said his ministry and the interior ministry would carry out an "extensive investigation" into the incident.

"It is clear that this reveals lapses in our (security) system," Interior Minister Claude Gueant told journalists.

"All the lessons must be learned so that the system can be even more watertight than it is today," he said.

Sarkozy denounced the activists for putting themselves and others at risk.

"It is quite irresponsible to take risks with one's life and the lives of others," he told a press conference.

"As far as nuclear security is concerned, we will absolutely publish all (security) audits," he said.

But Green Party leader Cecile Duflot thanked the activists for carrying out a "free audit" of security at French nuclear facilities.

The activists "proved the weaknesses of our (nuclear) stations," she said in a statement, adding: "Terrorist risks must finally be taken seriously by nuclear authorities and the government."

The incident comes as some in France have begun to question the country's long-held support for nuclear energy.

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

But the country's reliance on nuclear power has been increasingly called into question since the Fukushima disaster in Japan, which prompted Germany to announce plans to shut all of its reactors by the end of 2022.

Ahead of a presidential election next year, Socialist candidate Francois Hollande has agreed a deal with the country's Greens to push to reduce France's reliance on nuclear energy to 50 percent by shutting down 24 nuclear reactors by 2025.