The march, organized largely by the Left Front party and the Communists, comes before the French parliament's debate this week on a European fiscal treaty. Organizers put turnout at more than 80,000; Paris police declined to provide an estimate.
The protest exposed political pressures faced by President Francois Hollande — whose popularity has been sinking in recent polls — and notably from some ostensible allies of the left. It was the biggest demonstration of its kind since Hollande was elected in the spring, just weeks before his Socialists won control of the National Assembly.
The treaty would set up the European Stability Mechanism bailout fund that European leaders hope will help calm a debt crisis that threatens the eurozone and the global economy.
The main conservative opposition party and most Socialist lawmakers back the treaty. But it has splintered the French left: Far-left parties, the Greens and some dissident Socialists oppose it. The measure is nonetheless likely to pass in parliament because the two biggest parties generally favour it.
On Friday, the French government presented a budget that included high taxes on the wealthiest, but which critics say lacks fundamental reforms that could jumpstart economic growth.
Austerity has fanned recent violence in places like Spain and Greece.