He added that the authorities believed new terror attacks were being planned in France and other European countries.
Mr Valls also said 150 raids on suspected militants had been carried out across France early on Monday.
A total of 129 people died in the attacks on bars and restaurants, a concert hall and the Stade de France.
A huge manhunt is under way for surviving members and accomplices of the Islamist group that carried out the attack.
Police have named Brussels-born Salah Abdeslam, 26, as a key suspect. He was reportedly stopped by officers in the wake of the attacks – but then let go.
Meanwhile, French aircraft have attacked Raqqa, the stronghold in Syria of the Islamic State group, which has said it carried out the attacks.
Mr Valls said that France was dealing with a “terrorist army”, rather than a single terrorist group.
“We know that operations were being prepared and are still being prepared, not only against France but other European countries too,” he said, quoted by AFP news agency.
The prime minister said more than 150 raids on militant targets in different areas of France early on Monday.
Police sources told news agencies that properties in the Paris suburb of Bobigny, as well as the cities of Grenoble, Toulouse and Lyon, had been targeted.
Seven attackers died in the assault on the French capital, most of them after detonating suicide belts.
Suspected Paris attackers
- Salah Abdeslam, 26 – urgently sought by police
- Mohammed Abdeslam – reportedly arrested in Belgium
- Brahim Abdeslam, 31 – named as attacker who died near Bataclan concert hall
- Omar Ismail Mostefai, 29, from near Paris – died in attack on Bataclan
- Bilal Hadfi, 20 – named as attacker who died at Stade de France
- Four other attackers died during the assaults in the city
Salah Abdeslam, the man named by police as a key suspect, is said to have rented a VW Polo car that was found near the Bataclan concert hall, where 89 people died, and believed to have been used by attackers.
On Saturday he was in a vehicle with two other men near the Belgian border when it was stopped by police, officials said, but was released after checks.
Belgian police again stopped the car in the Molenbeek area of Brussels, but he was no longer inside, France’s Le Monde newspaper reported.
It is unclear whether the French authorities had matched the VW Polo found at the Bataclan venue to him at the time he was stopped.
Police have described Salah Abdeslam as dangerous, and warned people not to approach him.
One of the main lines of investigation concerns the Brussels suburb of Molenbeek. One of Abdeslam’s brothers, Mohammed, was reportedly arrested arrested there when he returned from Paris.
He remains in custody. Belgian police say they have made a total of seven arrests.
Belgium’s Premier Charles Michel said the Belgian authorities would crack down on Molenbeek, which has a reputation as being a haven for jihadists.
France is currently marking a second day of national mourning. A state of emergency declared by President Hollande remains in force. Thousands of extra police and troops are on the streets of Paris.
Source:BBC