Security forces in France have foiled attacks on a military installation with the arrests of four people allegedly linked to militant groups, President François Hollande said Wednesday. "This week, we stopped terrorist attacks that could have taken place," Hollande said on a visit to the southern coastal city of Marseille.

Minister of the Interior Bernard Cazeneuve said security forces staged dawn raids on Monday and arrested four people, aged between 16 and 23, who were "planning to commit a terrorist act" at a French military installation. The youngest was quickly released but the other three are suspected of planning to kidnap and behead a member of the military, possibly on Dec. 31 when the facility was thinly staffed. The oldest of the group served as a navy signalman at the base around the southern town of Collioure, which is also used for training by elite commando forces. He was identified as Djebril and was recently kicked out of the navy, a source close to the investigation who did not wish to be named said. The other key plotter was 17, and was already being closely watched by authorities due to his activities on social media and connections to French extremists in prison. All three of those still under arrest had been planning to travel to extremist-controlled areas of Syria, the security source said, but the 17-year-old's mother contacted authorities and he was interviewed by counter-terrorism officials.

No weapons were found during the arrests, the source said, although officials discovered documents on preparing explosives. The news of the arrests followed a statement from Hollande who said attacks had been thwarted in recent days.

France remains on high alert more than six months after the attack on a satirical magazine Charlie Hebdo, which claimed 17 lives. At traditional July 14 celebrations, Hollande said: "Every week, we are arresting, preventing ... terrorist acts." The suspected plotters are now in the custody of the French General Directorate for Interior security (DGSI) and anti-terrorist prosecutors in Paris have opened a probe, Cazeneuve said. "I want to congratulate our security services for this new blow to the terrorists and for again foiling an attack," the minister said. News of the foiled attack came just hours after two blasts on Tuesday at a petrochemical plant near Marseille, described by Cazeneuve as a "criminal act." Investigators have yet to pin down a motive for the explosions and there was currently "no link" with the foiled attack on the military base, he added.

The government says there are 1,850 French citizens or people living in France who are implicated in extremist networks, with around 500 in Syria or Iraq. Authorities have set up a hotline for friends or family concerned that someone could be tempted to join extremist groups, an effort that has yielded 2,500 leads. Following controversial counterterrorism laws passed last year, France is also preventing suspected extremists from leaving the country with some 118 travel bans having been enforced since the legislation was passed in November.