Nantes saw clashes between protesters and security forces. In Paris police fired rubber pellets on demonstrators outside the National Assembly.
Earlier, the cabinet approved using special powers to pass the changes without parliamentary approval.
Before, the French cabinet has given the go-ahead for Prime Minister Manuel Valls to force through highly controversial labour reforms.
An extraordinary cabinet meeting invoked the French constitution´s rarely used Article 49.3, allowing the government to bypass parliament. It came after rebel MPs from the governing Socialist party had vowed to vote down the bill.
The reforms will make it easier for employers to hire and fire workers.
The government says the reforms are essential to help cut high levels of unemployment. The changes make it easier for employers to hire and fire but opponents fear they will also enable employers to bypass workers´ rights on pay, overtime and breaks.
President Francois Hollande has faced months of resistance to the bill from students, unions and even members of his own Socialist Party.
Hundreds of demonstrators rallied outside the National Assembly in Paris on Tuesday, calling for President Hollande to resign, with the protests continuing into the night.
According to Le Parisien, the police used tear gas against protesters in Grenoble and Montpellier, reports from social media say. Lille, Tours and Marseille also saw demonstrations. In Toulouse two young protesters were injured in clashes with police.
The decision to invoke an article of the constitution to force through the reforms was made after the government failed to reach a compromise on the bill with a group of rebel MPs within the Socialist Party.
This tactic has only been used once before under President Hollande, again to push though disputed economic reforms.
Prime Minister Manuel Valls was booed by MPs from the far left and the conservative opposition when he announced the cabinet´s decision to the National Assembly.
The only way the bill can now be stopped is by the motion of censure - a vote of no confidence - that was filed by two right-wing parties on Tuesday.
Between them they have 226 of the 288 votes needed to topple the government on Thursday. However, correspondents say they are unlikely to find enough left-wing MPs willing to support them.