"The threat has never been as high. We have never had to face this kind of terrorism in our history," Valls told radio station France Inter Thursday.
A 24-year-old Franco-Algerian computer sciences student was arrested Sunday after police discovered an arsenal of weapons and detailed plans to attack a church in Villejuif, just outside the capital. They also found documents mentioning Al-Qaeda and the Islamic State group in his student accommodation.
Meanwhile, DNA has linked Sid Ahmed Ghlam to the murder of a woman who was found shot dead in the passenger seat of her car in Villejuif on Sunday, police say.
Ghlam had been known to intelligence services after posting messages on social media networks about his desire to fight alongside jihadists in Syria. But authorities who had made checks on him in 2014 and 2015 did not find anything to warrant further investigation, according to Interior Minister Bernard Cazeneuve.
The plot was discovered only after Ghlam called paramedics to treat what appeared to be an accidental self-inflicted gunshot wound to the leg. Kalashnikov rifles, handguns and bulletproof vests were then found by police in his car and at his student flat.
Paris prosecutor Francois Molins told French news agency AFP that there is evidence he "was in touch with another person who could be in Syria on how to carry out an attack, with the latter clearly asking him to target a church.”
Valls said Thursday that 178 Catholic religious sites have been placed under special protection, and the Archbishop of Paris Cardinal Andre Vingt-Trois met with the Interior Minister Bernard Cazeneuve Thursday morning to discuss his concerns.
"Even if we are constantly adapting the level of protection in the country, it doesn’t mean fear should spread over the entire country," Cazeneuve said after the meeting. "If that were the case, it would mean that we would have given in to the terrorists who want just that to happen. Instead we are determined to live freely."
French bishops also urged for calm.
"The terrorist threats, whatever they may be, aim to spread fear. Catholics will not give in," they said in a joint statement.
France has been on maximum alert following the January terror attacks on the offices of the Charlie Hebdo satirical weekly and a Jewish food shop, in which 17 people were killed.
The latest plot has emerged as controversial new spy laws are being debated in France's parliament.
Valls has defended the overhaul of intelligence legislation, saying it has been long overdue.