French riot police have used tear gas and water cannon after protests against a new labour law turned violent.
Unions are threatening fresh strikes unless the law is withdrawn
More than 160 people were arrested after clashes erupted in eastern Paris following a day of largely peaceful demonstrations across France.
Vehicles were set on fire and stores were damaged as masked youths clashed with police.
Twenty-four people, including seven police officers, were injured in the violence, which lasted about six hours.
A tense calm had returned to the streets of the French capital by Sunday morning.
Cars set alight
The violence broke out at the eastern Place de la Nation as police attempted to disperse demonstrators following a mainly peaceful march through the capital involving students, workers, pensioners and families.
Demonstrators hurled stones and bottles at officers, who eventually drove them back, charging the crowd and using tear gas grenades.
Several cars were set on fire and nearby shop windows smashed.
About 500 students then marched on Paris' Sorbonne university in the Latin Quarter, chanting: "Liberate the Sorbonne."
Students removed some of the barriers erected by police to block access to the university, and threw a fire bomb at a riot police van, but a fire was quickly extinguished. Police repelled the students with water cannon.
Clashes also erupted in other cities, including the port of Marseille, where demonstrators tried to set fire to the entrance to the town hall. One officer was injured and six youths were arrested, police said.
'Message from the street'
Unions said 1.5 million demonstrators took part in more than 150 rallies across the country against the government's First Employment Contract (CPE).
The interior ministry put the overall turnout at just over 500,000, but unions urged Prime Minister Dominique de Villepin to heed "the message from the street".
The government will be counting the political cost of the violence
Protesters are bitterly opposed to the new law, which allows employers to end job contracts for under-26s at any time during a two-year trial period without having to offer an explanation or give prior warning.
The government says it will encourage employers to hire young people but students fear it will erode job stability in a country where more than 20% of 18 to 25-year-olds are unemployed - more than twice the national average.
The demonstrations came after a series of mass protests by students in dozens of French universities, which have severely disrupted classes.
Several unions have threatened fresh strikes next week unless the government withdraws the law, AFP news agency reported.
The BBC's Caroline Wyatt in Paris says the protests have gained their own momentum, turning into the worst crisis that Mr de Villepin has had to face since taking office last year.
The contracts have been seen by students and many on the left as an attack on job security at a time when many in France are feeling deeply threatened by globalisation and any hint of change at home.
An official spokesman has talked of the need to improve the contracts - which sounds like a government preparing to soften its position in the face of growing disruption, our correspondent adds.
Mr de Villepin proposed the CPE law as part of a series of measures designed to help youths in the French suburbs who took to the streets last year.