Demonstrators have pelted police officers with bottles, scooters and garbage bins have been upturned, and at least three people have been arrested as labour protests turned violent on Thursday.
The CGT union said 50,000 people demonstrated in Paris.
The protesters have slammed the government as being "undemocratic" after it pushed through its unpopular raft of labour reforms without a vote in parliament on Tuesday.
The already unpopular Socialist government faces a no-confidence vote on Thursday, after its labour bill led to two months of demonstrations already.
Unions had called for the demonstrations to coincide with the vote of no confidence.
Paris saw a march from Denfert-Rochereau in the 14th arrondissement to Les Invalides on the south bank of the River Seine.
Some protesters attacked security forces with baseball bats, reported BFMTV, to which officers responded with tear gas.
Police released a statement saying that officers were "attacked with Molotov cocktails, bottles, stones, and projectiles at them".
"These were accompanied by insults and anti-cop slogans," the police added.
The troubles weren't confined to Paris either, with reports of tense situations in Nantes to the west and Toulouse to south.
The government bypassed parliament on Tuesday to force through the highly contested reform.
"Pursuing the debate in parliament would pose the risk of... abandoning the compromise that we have built," Prime Minister Manuel Valls told parliament at the time.
The government says the reform will give companies more flexibility to fight endemic unemployment, which stands at more than 10 percent. Joblessness has been the defining issue of Hollande's four years in power.
Opponents argue that the bill will only serve to erode job security.