French far-left leader Jean-Luc Melenchon has rallied disaffected voters against President Emmanuel Macron's plan to weaken worker protections.
It comes amid spreading discontent with the president's vision of a more business-friendly economy.
Thousands gathered at the iconic Place de la Bastille for a march through eastern Paris, including Melenchon supporters, environmental activists, anti-capitalism campaigners and others just expressing frustration with France's new, young president.
Mr Melenchon's Defiant France party said it brought in protesters by bus from more than 150 French towns.
A huge balloon on top of a truck read: "Macron, Resign!"
The protesters' anger is centered on labour decrees that Mr Macron signed on Friday reducing French unions' influence over workplace rules and making it easier for companies to fire workers.
Mr Macron says the change is needed so France can compete in the global economy. Opponents say he is dismantling the French lifestyle.
Mr Melenchon, seeking to position himself as France's leading opposition figure after a strong fourth-place showing in this year's presidential election, wants the labour law decrees withdrawn.
The protesters are also angry at what Mr Melenchon calls the "authoritarian" way Mr Macron imposed the reforms.
He used a special procedure allowing the government to change labour law by executive order instead of going through a lengthy debate to pass a bill in parliament.
The president said during the signing ceremony on Friday that the first labour measures will start being applied next week, and all will be implemented by the end of the year.
Among the most contested reforms is one that caps the financial penalties awarded by courts for wrongful dismissals.
Another eases regulations governing when and why companies can dismiss workers.
Mr Macron lauded the "unprecedented wave of changes" to France's social model, along with changes to unemployment benefits and a training plan for jobless people that will be set up next year.
While he shone at the UN General Assembly in New York last week and has made a strong mark on the international stage, he has struggled with myriad critics at home.
Farmers, riot police and carnival workers have also held protests in recent weeks over work policy changes under Mr Macron, and truckers plan road blockades on Monday.