The rally, which spanned several street lengths, was organized by union groups opposed to the changes.
The march ended at the Montreal office of Premier Philippe Couillard.
Marc Ranger, the leader of the union coalition, says he believes the protest movement will grow once the rest of the population realizes the extent of the government's austerity measures.
He says the pension reforms are part of a larger cost-cutting program that will effect health care and social services.
The legislation proposed by Couillard's government calls for a 50-50 split between municipalities and unionized workers on pension contributions and deficits.
It would also allow municipalities to temporarily suspend cost-of-living hikes.