More than 100 demonstrators were detained and 24 security officers injured as clashes broke out during protests across France against planned labor law reforms on Thursday, the interior minister said.
Striking workers burned tires and tens of thousands of protesters marched through the streets in Paris, Rennes, Nantes, Marseille and other cities.
Interior Minister Bernard Cazeneuve said three officers had been badly hurt in clashes in the capital, and one was in a critical condition.
France's powerful CGT union says the legislation, due to be debated in parliament next week, will let employers bypass regulations on basic worker rights by giving bosses greater freedom to set terms of pay, rest and overtime rates.
"We want it withdrawn as long as the goal means the law is no longer the rule, and that every company can opt out on work time or overtime rates.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.
Labour Minister Myriam El Khomri has already diluted the text to drop reforms such as a cap on financial settlements in cases of unfair dismissal.
With traditional Labour Day rallies organized for Sunday May 1, Hollande faces the risk of a more broad-based protest movement coalescing.
An opinion poll published by BFM TV showed close to 80 percent of French people fear an escalation, despite news this week of a significant drop in the monthly jobless count.
France's Labour Ministry reported the steepest fall since the economic boom days of 2000, in a rare boost for Hollande.
At Orly airport, France's second largest, airlines were told on Thursday to scrap one in five flights although traffic was reported as largely normal at the capital's main Charles de Gaulle international airport.