Montreal was bracing for another night of unrest Thursday as student protesters massed in the city core.

Police declared Thursday's protest illegal, as up to 2,000 students marched on streets near McGill University and through the downtown area.

The declaration was made after protesters hurled projectiles at police patrolling the march, CTV Montreal reported.

Despite the police presence, the demonstrators continued to chant "We haven't done anything," as nearby officers kept an eye on the crowd.

On Wednesday night, a riot in the city's downtown led to the arrest of 85 people and left scores of businesses and one police station damaged.

Police said that protesters employed "Black Bloc" tactics during Wednesday's demonstration. Three police officers were injured in the melee.

But unlike Wednesday's protest, many taking part were not covering their faces.

The demonstrations, which appear to have spread from students to include a range of other activist groups, follow an abrupt halt to negotiations over proposed tuition hikes in the province.

There have been 160 protests in the 11 weeks since students walked out of classes, and Wednesday's destruction marked an upswing in hostilities.

The story has made headlines internationally. CNN showed dramatic images of the riot on U.S. national television.

Montreal Mayor Gerald Tremblay pleaded for resolution Thursday, and cited dangerous recent events as evidence that things have gone too far.

Tremblay said that bricks were thrown in Montreal's subway system, and that rocks were tossed from an overpass in the city's downtown.

"Does a tragedy have to happen?" Tremblay said.

"Montrealers ... are fed up. They don't want to go through this. This stuff always happens in Montreal. It's the same thing for the businesses affected ... It's the same thing for Montreal's reputation on the world stage."

The violence erupted several hours after an abrupt end to the negotiations Wednesday, with the government accusing one student group of violating a truce and condoning vandalism.

Lt. Ian Lafreniere, of the Montreal Police, said police declared the protest to be an illegal demonstration at 10:15 p.m., issued multiple warnings to the crowd, then moved in and began making arrests.

He said 15 people were arrested before 10:15, and 70 more were arrested around 1 a.m. -- after 15 warnings were issued by police.

"We've got to be careful because I'm not talking about students this morning, I'm talking about people with criminal intentions, these people were committing crimes, mischief...they were people taking advantage of the situation," he said.

He said many protesters used Black Bloc tactics, wearing black clothing, masking their faces, acting as a group and using weapons they had hidden along the protest route.

Lafreniere said the violent protesters have taken away from the message of legitimate demonstrators concerned with rising tuition in Quebec.

"We're not talking about the message this morning, we're talking about how business owners and residents got their vehicles smashed," he said.

Banks and other businesses, cars, even a police station had windows shattered by an angry mob that spilled out from a larger crowd of thousands of student protesters.

The violence followed a breakdown in talks between the two parties to end an 11-week fight over tuition hikes. Education Minister Line Beauchamp steadfastly refuses to back down on plans to hike tuition by $325 a year over five years.

The government said one of the student groups, C.L.A.S.S.E., broke the truce by protesting Tuesday and condoning violence on its website and booted it from talks Wednesday.

"You can't play both sides," Education Minister Line Beauchamp said. "I regret that this (group) has chosen its camp."

That prompted two other students groups to walk out in protest leading to the street demonstrations and subsequent riot.

The leader of one student group said the talks are suspended until the banned group is invited back to sit at the table again.

Martine Desjardins, of the Quebec Federation of University Students, said early discussions revolved around issues like student aid and not tuition fees.

"People want us to talk about the tuition fee problems," she said.

"We were ready to spend hours and more hours on this topic," Desjardins said.

Although pressure is mounting on the Liberal government of Premier Jean Charest to back down from the hike, it appears less likely as the party rises in the polls, raising the possibility of a snap spring election.

Polls indicate Quebecers generally support the fees.

One survey this week showed the poll-leading Parti Quebecois, which has staunchly endorsed the students, losing support and seeing its lead evaporate.

A provincial election must be called between this spring and late 2013.

As the talks broke down Wednesday, students took a parting shot at Beauchamps, calling her a scolding school master more interested in political grandstanding than negotiating.

It was moments after the talks ended when students began spilling into the streets of Montreal and Quebec City, joined later by thousands of others denouncing Charest and demanding an election.