Police fired water cannon in the south of the city to quell rioters as "several hundred" masked protesters lobbed objects at security forces in bloody scenes in the French capital.
Police said 29 members of the security forces were among the 40 injured overall.
The clashes erupted as the international spotlight was on France as the host of the Euro 2016 football championships, which have also been marred by violence between fans.
Strikes closed the Eiffel Tower and disrupted transport links as tens of thousands of fans continued to pour into the country for Europe's showcase football event.
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Protesters shouting "Paris, rise up" and "everyone hates the police" also smashed shop windows and targeted banks in running battles through Paris with police.
An AFP photographer saw one man being led away by officers in riot gear with blood dripping from a wound above his eye onto his white T-shirt.
The strike is the latest in months of industrial action that has caused severe disruptions to air and rail transport, exhausted fuel supplies and led to mountains of rubbish on the streets of Paris.
"I want the reforms to be withdrawn, pure and simple. Only then will it stop. For the government's sake, they should withdraw the law, otherwise we'll block the economy."
With France on high alert over fears of attacks during Euro 2016, overstretched security forces had feared the demonstrations could turn violent and banned 130 known troublemakers from taking part.
The terrorism threat was thrust back into the spotlight after a man claiming allegiance to the Islamic State group stabbed to death a police couple at their home in a northwestern Paris suburb late Monday.
President Francois Hollande's Socialist government has voiced hope the latest day of protest would be a last stand for the movement.