A demonstration against the economic policies of Premier Philippe Couillard's government began in the city's business district at around 1 p.m. with several thousand people from unions, student federations and social groups.
The protest was largely peaceful but by late afternoon a small group of demonstrators who hadn't gone home began confronting police, setting up barricades on certain streets and throwing objects at officers.
Authorities responded by firing tear gas in scenes similar to others witnessed in previous protests in recent years.
Many businesses hired security guards for Thursday's protest and locked their doors as a precaution.
Protesters came to Montreal from across the province with the goal of pressuring the provincial government to backtrack on various cost-cutting measures.
Members of the city's firefighters union also attended, as did hospital workers and civil servants.
Demonstrations against the Liberals have multiplied in recent months as Couillard's government continues with its push to reach a balanced budget in 2015-16.
Camille Godbout, a spokeswoman for the ASSE student federation, which helped organize the protest, said 135,000 students were on strike from their classes to decry the government's recent budget.
"135,000 students on strike — that's something," she said. "We want to force the government to act.
"We will continue to mobilize."