Police said up to 12,000 people took part in Tuesday’s march, while unions put the number at about 55,000.
The marches have turned violent in some cases, with clashes breaking out between protesters and police in the western cities of Nantes and Rennes.
The French government says the changes are needed to boost economic growth and decrease unemployment.
They include measures aimed at easing regulations on working hours and changing the rules governing dismissal compensation.
Polls show as many as three in four people oppose the overhaul, fearing the planned legislation could weaken workers’ rights and protections.
French President Francois Hollande said he would pursue the changes regardless.
“This law, which is being debated, including on the street, will pass,” he told Europe 1 radio on Tuesday.
He said “too many governments have given in” in the past, creating the conditions that have hindered France’s attempts to boost economic growth.
France has faced discontent over the past two months after a new left-wing youth movement, La Nuit Debout, which stands for “rise up at night”, swept across the country in protest against labour law changes as well as inequality and the environment.
The movement has spilled over into Belgium, Germany and Spain, with calls for broader street protests next week and strikes by railway workers, dockers, truckers and airport and refinery workers.