Molotov cocktails were tossed during a large protest designed as an act of defiance Friday against a legal crackdown by the Quebec government.
At least one explosive device was lobbed over a downtown intersection, sailing through the air before it crashed into the street in front of police.
It erupted in flames and a puff of smoke. A Canadian Press photographer reported seeing at least two such objects thrown and there were other reports of multiple devices being tossed at that spot on the edge of Chinatown.
Police responded to being pelted with projectiles by firing rubber bullets, noise bombs and tear gas into the crowd. One person was arrested for assaulting an officer.
"Criminal acts were committed," the police said in its Twitter feed. "(The protest) has been declared illegal. We asked people to disperse immediately."
The protest had been peaceful over its first hour.
Participants had said they were encouraged by the thousands-strong turnout and by the support from bystanders honking their car horns or clapping while they passed.
Opponents of the Charest government — including its political opposition — have begun attacking its approach to the protests, even more than the tuition hikes that originally sparked the unrest.
The Montreal march came hours after the government passed a law setting limits on the ability to protest. Also Friday, a new municipal bylaw imposed stiff fines on people wearing masks while demonstrating.
A few people in the crowd were in fact wearing masks. However, the mask bylaw and the protest limits were not expected to be applied until Saturday.
Police said they were given the march route in advance — one of the many stipulations of the new provincial law. They were not commenting, however, on reports that their phone lines were flooded by people calling to report the route, in a unique gesture of civil disobedience.
The nighttime protests have been going on every night for nearly a month. But some participants said this was their first one.
Milly Pominville, a 20-year-old junior college student, acknowledged she was nervous attending the protest after the passage of the special law.
"I don't want anything bad to happen," she said.
"But I hate Jean Charest so much. He has to go."
Ms. Pominville called the new law "stupid" and vowed to return for the marches every day from now on.