Crowds gathered at Parc Émilie-Gamelin at 9 p.m. before marching along the downtown streets.
Police quickly declared the protest illegal, saying an itinerary of the route was not provided, and began making arrests.
There were clashes between protesters and police near the corner of Rene Levesque and St.-Laurent.
Police said some projectiles were thrown and the windows on two patrol cars were smashed.
The event was organized by an anti-capitalist group called Mouvement étudiant révolutionnaire.