“The threat has never been so high,” said French Prime Minister Manuel Valls as he announced police had thwarted five terror attacks in recent months.
He said the latest plot centered on churches in the southern Parisian suburb of Villejuif.
Sid Ahmed Ghlam, a 24-year-old electronics student from Algeria, has been arrested, suspected of planning the attacks.
He was detained on Sunday after apparently accidentally shooting himself and calling an ambulance.
According to officials, he was found in possession of a number of Kalashnikovs, hand guns and ammunition.
Police are also said to have found further weapons and bullet-proof vest in the suspect’s car, as well as literature on al-Qaeda and the ISIL militant group.
Ghlam is also thought to be linked to the fatal shooting in Villejuif of 32-year-old Aurélie Châtelain.
Traces of her blood were found on one of his jackets, according to prosecutors. Reports suggest Ghlam may have shot Châtelain as he attempted to steal her car.
French Interior Minister, Bernard Cazeneuve, said the suspect was known to authorities and had expressed a desire to travel to Syria to fight with extremists.