The demonstrators gathered at Bastille and are expected to march to Opera.
Doctors and lawyers marched in Paris on Monday against French President Emmanuel Macron's pension reforms.
The demonstration, which went from Bastille to Opera, was the latest in a wave of protests against Macron's signature reform: streamlining France's complex and expensive pension system that allows some French workers to reture as young as 50.
It has led to over 60 days of strikes and protests, includng by transport workers, women's groups, and those in the tourism and energy sectors, which has seen tourist attractions like the Eiffel Tower closed and electricity and gas cut off by workers at energy plants.
But Macron has stuck firm to his reform, hoping that he can force it through parliament in February despite ongoing industrial action and widespread opposition. Polls suggest that France is divided on the reform, with older workers and young people opposed.
What is the reform?
The plan will streamline France's 42 retirement systems into a single point-based system for all workers, public and private sector alike, and abolish special provisions for some workers. It will also introduce a minimum pension.
The new point system will come into place starting in 2022 for the youngest workers, and then gradually for older workers. It will only affect those born after 1974.
The government backed down earlier this month on plans to raise the age to receive a full pension to 64, at least for now. Instead, Prime Minister Edouard Philippe agreed to negotiate with unions starting next week on a way to make the new retirement system financially sustainable — and that's likely to require eventually raising the retirement age.