Trade unions have staged protests across France against an overhaul of labour rules expected to be passed by parliament.
The lower house of parliament, where President Francois Hollande's Socialist government has a slight majority, is expected to pass his measures loosening firing and hiring rules on Tuesday, opening the way for a Senate vote on April 17.
Some trade unions, led by the leftwing CGT and backed by hard-left allies in parliament, are determined to stir up opposition against what they called a "traitorous" bill, with marches in up to 170 towns and cities.
In the Mediterranean city of Marseille, thousands marched on Tuesday bearing banners with the words "No to breaking the labour code", seen as the most comprehensive labour reforms since World War II.
"We won't let anything past us," Mireille Chessa, regional CGT chief, said.
Accusations from the left that Hollande has abandoned socialism could weigh on his already dismal approval ratings, close to 30 percent, and hurt his party's performance in municipal elections next year.
Left-wingers are already finding some of the president's economic policies, such as raising sales tax to fund a reduction in company labour costs, hard to swallow.
And he came under fresh fire this week after his former budget minister admitted he had lied for months about the existence of a secret foreign bank account.
Public opinion on the reforms is divided.
But a survey by pollster BVA in March found that 62 percent of respondents supported passing the bill, making it more popular with the French than Hollande himself.
"The point is ... to make workers aware of the impact this is going to have on their daily lives," Thierry Lepaon, head of the CGT union, told Canal+ television.
Nationwide protests last month against the labour bill drew 200,000 participants, according to a CGT estimate, a modest
turnout explained by the fact that members of the moderate CFDT union did not participate.
France's two largest unions have been at odds since January when the CFDT, backed by two allies, approved the reforms while the CGT refused.
In dozens of articles, the bill aims to inject more flexibility into a system the IMF, OECD and ratings agencies say is to blame for chronically high unemployment because it gives some workers too much job security and others too little.
Pressure from Brussels to fix structural brakes on competitiveness, from rigid labour markets to tax fraud, compelled leaders in Italy and Spain to undertake similar sweeping reforms despite opposition on the streets.
However, months after the reforms were passed, unemployment in Spain remains above 25 percent and above 11 percent in Italy.
Michel Sapin, labour minister, has said the changes will be implemented as soon as possible in a bid to combat unemployment, which has risen steadily over the past two years to reach 10.6 percent, close to an all-time record in jobless claims.
But hard-left lawmakers, who have sought minor amendments to weaken the bill, hope to slow its passage through the Senate.
"We are acting as amplifiers for the demands of the protesters today," Andre Chassaigne, head of the Communist Party's group in parliament, told left-wing paper L'Humanite.