Getty ImagesVehicles were set alight outside several French prisons, and one jail was hit by gunfire from an automatic weapon, in a wave of overnight attacks.
Justice Minister Gérald Darmanin said the jails had faced "intimidation attempts" and linked the attacks to the government's crackdown on drug trafficking.
Seven establishments were targeted, according to the Parisien newspaper: in Toulon, Aix-En-Provence, Marseille, Valence and Nîmes in southern France, and in Villepinte and Nanterre, near Paris.
France's national anti-terrorism prosecutor's office said it was investigating.
In a post on X, Darmanin said he was travelling to support affected officers at the prison in Toulon, which was targeted with gunfire.
Without directly attributing blame for the attacks, he said the French government was "facing up to the problem of drug trafficking" and taking measures that would "profoundly disrupt" criminal networks.
Interior Minister Bruno Retailleau said the government's response must be "relentless".
"Those who attack prisons and officers deserve to be locked up in those prisons and monitored by those officers," he posted on X.
He added that he had instructed police to immediately strengthen security at prison facilities.
The prison guard union, FO Justice, expressed its "deepest concern and anger" following the "extremely serious" attacks overnight.
The union posted updates from the aftermath of several attacks on X, including images of burnt-out vehicles in prison car parks and bullet holes in the Toulon prison entrance gate.
It called for urgent government action to protect prison staff.
Monday night's attacks come after seven vehicles were set on fire in a similar attack on France's national school of prison administration on Sunday, according to the union.
"It is worrying to note that some people no longer hesitate to directly attack the prison's property, a symbol of state authority," it said in a statement.
No group has claimed responsibility for the attacks, but the Parisien reports that the letters DDPF - meaning "French prisoners' rights" - were found inscribed on damaged vehicles. The AFP news agency says anarchist slogans were found at some sites.
AFP quotes a source close to the case as saying the attacks appeared to be coordinated and "clearly linked" to the government's strategy against drug trafficking.
A law is passing through the French parliament which creates a special prosecutor's office to deal with drugs crime, with new powers for investigators.
