Striking workers burned tyres, threw bottles and stones at police; tens of thousands of protesters marched through the streets in Paris, Rennes, Nantes, Marseille and other cities.
Interior Minister Bernard Cazeneuve said protest organizers should "condemn these excesses by a handful of hooligans".
Three officers had been badly hurt in clashes in the capital, and one was in a critical condition, he said
According to France´s powerful CGT union, the proposed law, which will be debated in parliament next week, would remove some of the protection workers enjoy against being laid off, in a bid to encourage businesses to hire more people.
"We want it withdrawn as long as the goal means the law is no longer the rule, and that every company can opt out on work time or overtime rates. That´s unacceptable," CGT chief Philippe Martinez said.
Trade unions say the proposed legislation is not the way to address an unemployment rate which President Francois Hollande promised to haul down but which has remained stubbornly above 10 percent.
Deeply unpopular, Hollande faces a testing few months against a backdrop of protests and sluggish economic growth, before he announces whether he will contest next year´s presidential election or not.
There have been repeated protests against the proposed changes over the past two months.