President Emmanuel Macron's plan to overhaul the country's complex labour code, fulfilling a central campaign promise, sparked the first strikes and protests Tuesday.
While the 39-year-old centrist believes that making the labour market more flexible will help drive down unemployment of 9.5 percent, opponents fear an erosion of worker protections.
What he hopes will be a signature reform entails a major overhaul of the more than 3,000-page labour code which sets out workers' rights, with some measures dating back over a century.
Macron, whose Republic on the Move party enjoys a comfortable majority in parliament, intends to fast-track the reforms by executive orders which are expected to be ratified by parliament in the next few months.
The reforms will give small companies in particular more freedom to negotiate working conditions with their employees, rather than being bound by industry-wide collective agreements negotiated by trade unions.
A cap has also been set on the amount of compensation awarded by industrial courts in cases of unfair dismissal -- a key demand of bosses who complain that lengthy and costly court cases discourage them from hiring.
Other measures include streamlining workers' committees, which are mandatory within large companies, and expanding the use of flexible "project contracts" which allow companies to hire people for a specific job.
In a further concession to companies, multinationals whose French operations are struggling will find it easier to lay off staff, while workers made redundant will receive higher payouts. Several unions also fear new measures that would give employers greater leeway to launch voluntary redundancy plans.
Philippe Martinez, the head of the Communist-backed CGT union leading Tuesday's protests, said the reforms give "full powers to employers" and has called for strikes and rallies.
Another day of demonstrations has been organised by the CGT on September 21.
But the leaders of more moderate unions, including the CFDT -- the biggest private-sector union -- and the hard-left Force Ouvriere have adopted a wait-and-see approach.
"We need to stop thinking that trade union action only makes sense when we demonstrate," the head of the CFDT, Laurent Berger, told Franceinfo radio on Tuesday, explaining how he favoured dialogue.
In parliament, the opposition to the changes is being led by the radical France Unbowed party of leftist firebrand Jean-Luc Melenchon, which is planning a mass march in Paris on September 23.
The right and centre-left parties in parliament have broadly backed the reforms.
- Will the protesters succeed? -
Hundreds of thousands of people demonstrated last year against plans by former president Francois Hollande to push through another round of business-friendly reforms to labour law.
The union-led action forced the government to water down their initial proposals, but despite months of strikes and demonstrations most of the measures cleared parliament and became law.