France was disrupted by a third straight day of strikes and demonstrations on Thursday as the prime minister called for protesters who torched a police car to face "harsh" punishment.
Rail staff and air traffic controllers downed tools in the latest mass protests against the government's labour market reforms, causing dozens of flights and trains to be cancelled.
Prime Minister Manuel Valls threatened to use force to break up protests which interfered with the operation of ports, refineries and airports.
But he reserved his strongest words for those who attacked a police car in Paris on Wednesday on the sidelines of a rare rally by police officers protesting against "anti-cop hatred".
With a policeman and a policewoman still inside the car, a small group of masked protesters hammered the car with iron bars before hurling in an explosive device, causing it to burst into flames.
The two officers managed to escape unharmed.
Valls said: "The punishment must be harsh. The inquiry has just started, arrests have been made."
Four suspects were arrested on Wednesday and a fifth was picked up Thursday, sources close to the investigation said.
Paris police chief Michel Cadot said the officers "were in the car when the explosive device was thrown inside" and the account was confirmed by a surveillance video seen by AFP.
Violence has erupted at several demonstrations in recent weeks -- mostly against the controversial labour reforms forced through parliament by the deeply-unpopular government of President Francois Hollande.
Small groups of troublemakers appear to have infiltrated the protests, bent on attacking security forces.
Valls called on demonstration organisers to prevent troublemakers -- known in French as "casseurs" -- from mingling with the crowd.
But his call for organisers to think twice before staging a protest was dismissed by CGT union chief Philippe Martinez.
"You cannot prevent democracy from being expressed just because there are problems on the sidelines of demonstrations," he said.
Over the past two months, some 350 members of the security forces have been injured during protests against the proposed labour reforms, which were forced through the lower house of parliament last week without a vote.
The national rail operator SNCF said around 14 percent of its staff had gone on strike Thursday, while Orly airport -- the second-largest in Paris -- was forced to cancel 15 percent of its flights.
The labour reforms have sparked two months of protests on France's streets, drawing 68,000 nationwide on Tuesday, police said, while organisers put the turnout at 220,000.
The government says the changes contained in the draft law will make France's notoriously-rigid labour market more flexible and create jobs.
But opponents say it will erode job security and do little to bring down the unemployment rate, stuck at 10 percent and nearly 25 percent for young people.
The labour reform, which would make it easier for employers to hire and fire workers, is likely the last major piece of legislation to be put forward by Hollande's government.
Hollande, whose poll ratings are among the lowest of any post-war French president, faces an election race next May.
He has said he will decide by the end of the year whether he will run, but he said this week he saw no other alternative on the left.