Thousands of people have taken part in demonstrations across France against the centre-right government's plan to reform the 35-hour week.
A majority of French people support the protests
Police estimated about 300,000 people took part in marches across France.
The unions held three days of strikes in the public sector last month, and believe there is a growing mood to challenge the government.
The government says the 35-hour week has raised labour costs and kept unemployment high.
The largest demonstration in Paris was headed by trade union leaders as well as by prominent figures in the opposition Socialist party.
At least 12,000 protestors took to the streets in Toulouse, in south-west France, police said, although organisers said at least 20,000 took part.
There were also demonstrations in Rennes, Bordeaux, Lyon, Metz and Saint-Etienne.
Demonstrators have been buoyed by a recent poll showing that at least 69% of the French public supports the current protests.
The demonstrations came as a bill to enable private-sector employees to opt for longer hours makes its way through the French parliament.
Under the proposed law, the number of overtime hours employees can work each year will be increased from a 180 to 220.
However, the changes would not affect the public sector, which employs nearly a quarter of all French workers.
Union leaders accuse the government of trying to turn back the clock.
They fear this is the first step towards abolishing the 35-hour week, says the BBC's Caroline Wyatt in Paris.
The 35-hour working week was introduced in France by the country's last Socialist government.
But Jacques Chirac's centre-right government blames it for putting up the cost of labour and keeping unemployment at a stubbornly high 10% of the French workforce.