Firebrand French leftist Jean-Luc Melenchon on Saturday vowed a lengthy battle against the labour law changes pushed through by President Emmanuel Macron as a tens of thousands of people took to the streets to demonstrate.
The leader of the hard left France Unbowed party had appealed to opponents of the changes to the labour code to "swamp" Paris in order to try and force Macron to back down on his signature reform.
The police estimated that 30,000 people took part in the third mass protest in two weeks against what Melenchon called Macron's "offensive against the French people".
The organisers estimated 150,000.
Addressing the crowd a day after Macron signed the reform into law, his opponent-in-chief declared: "The battle is not over. It's only beginning".
Melenchon, 66, accused the centrist Macron, 39, of fuelling a "race to the bottom" by giving businesses more freedom to negotiate conditions with workers.
"The work contract will no longer be a safeguard but a sort of paper rag," he warned.
- Street versus ballot box -
Former investment banker Macron argues that the changes -- which also make it easier to fire workers while giving higher payouts to those made redundant - will help bring down stubbornly high unemployment of 9.6 percent.
He fast-tracked the measures, which were eagerly awaited by the business community and France's EU partners, using executive orders that gave parliament almost no input.
The changes chip into worker protections that have long been sacrosanct in France, frustrating reform-minded governments whether on the left or the right.
The left has also come out swinging against his plans to cut housing subsidies and reduce the scope of a wealth tax, claiming it proves he is a right-winger.
"France has never had so many billionaires and millionaires. Why is it always the workers who have to tighten their belts?", Louis Bousquet, a jobless 33-year-old who travelled from the central Creuse region for Saturday's protest, complained.
Leaning on a stick, 48-year-old Zabou Hervieu, told AFP she was worried for her 10-year-old daughter.
"I'm here for her future. Will there still be real jobs with real salaries when she grows up," she wondered.
Macron has shrugged off the protests, insisting he has a mandate for change after his presidential win in May and his party's thumping parliamentary victory in June.
"Emmanuel Macron sees the election as a blank cheque to do what he wants... He has an authoritarian approach to power," David Guiraud, France Unbowed's 24-year-old spokesman on youth issues, told AFP.
Melenchon, a lawmaker who placed fourth in the presidential election, has seized on the disarray in the rudderless mainstream left and right to present his France Unbowed as the only real opposition -- both in parliament and on the street.
"It's the street that toppled France's kings, it's the street that drove out the Nazis," Melenchon, a consummate campaigner, said Saturday.
- 'Record' fast reform -
The resistance has, however, been weaker than that faced by Macron's Socialist predecessor Francois Hollande last year over his changes to the labour code, which sparked months of sometimes violent protests.
On Thursday, some 132,000 people demonstrated across France, down from around twice that number a week earlier.
Addressing reporters at the Elysee Palace after signing five executive orders before television cameras Macron boasted he had implemented his campaign pledge to shake up labour relations "in record time".
Polls show his approval ratings plunging as he moves ahead with his programme of tax and spending cuts, with only around 40 percent of voters declaring themselves satisfied.
His characterisation of opponents of his reforms as "slackers" has become a rallying cry, with protesters coining slogans such as "Slackers of the world, unite!"
Philippe Braud, professor emeritus at Paris's Sciences Po university, said he believed popularity was not currently a concern for Macron.
"He knows he won't be defeated in the street," Braud said.