Police arrested 84 people, including 27 on criminal charges, and used tear gas to disperse demonstrators at a night-time protest in downtown Montreal organized by a local anti-capitalist group called CLAC.

Hundreds of people gathered at at least three different locations around the city, well before the protests officially got underway at 7 p.m. ET.

The group called on protesters to cause the "maximum disruption possible" to businesses downtown.

However, the majority of the crowd were people carrying anti-austerity messages with no intention of vandalism or violence.

By 7:10 p.m., police had already declared the demonstrations illegal and began ordering protesters to disperse.

As demonstrators moved up McGill College Avenue from de Maisonneuve Boulevard, police began launching rounds of tear gas.

Children gassed by police

Families and bystanders were among those gassed early in the demonstration. One woman said she brought her family to the protest thinking it would be peaceful.

Within an hour of the May Day protest beginning, Montreal police started firing tear gas cannisters into the crowd. (CBC)

"Suddenly, there was tear gas all around us," the woman, who did not give her name, said, as one of her children cried in the background.

"We have a baby with us."

Montreal police Sgt. Laurent Gingras said it's regrettable that families were caught in the fray, but he said police warned people to leave the area before firing rounds of tear gas.

Montreal police confirmed there were arrests, but did not have a final tally.

Police kettled protesters

Gingras said one of the challenges police face was splinter groups that appeared across the city, making it difficult to disperse protesters.

Bystander Sarah Campbell was walking home from eating dinner near Sherbrooke Street and Hutchison Street when she saw police approach one of the splinter groups, comprised of around 15 protesters.

She said as many as 11 police cars appeared. Police kettled them, spraying one with pepper spray, and punching another in the face.

"It was surreal, almost. I really felt not scared, but upset. Well no, I was scared," she said.

Police car damaged

One officer suffered facial cuts and was taken to hospital with injuries that were not considered life-threatening, said Gingras.

"Police cruisers and cars belonging to ordinary people were damaged," he said.

One protester was seen smashing the windows of a cruiser with his fists.

Several people were sent to hospital, including two police officers – one for facial injuries and a second for back injuries, after police said that officer was beaten with a stick.

84 arrested, including 27 on criminal charges

In all, 84 people were arrested by the time the demonstration wrapped up at around 11 p.m.

Fifty-seven were intercepted under a municipal bylaw that forbids protesting once police declare the gathering illegal.

Twenty-seven face criminal charges, including 10 arrests for assault of a police officer, assault with a weapon and disarming a police officer.

Earlier May Day protests peaceful

The protest was in stark contrast to those held earlier in the day when thousands took to the streets of Montreal and elsewhere in the province in peaceful demonstrations.

Those protests focused primarily on big banks and the Quebec Liberal government's cost-cutting measures.

Alexa Conradi, president of the Quebec Federation of Women, said the Couillard government hasn't taken into account any of the proposals presented by social groups.

"Instead, what it's doing is causing cuts to education...in public services, in health care and in the regions." she said.

"A lot of these services are absolutely essential to maintaining a degree of equality in our society."

Conradi added that various groups decided to protest throughout the province because of what she called the government's refusal to listen to them.

The Montreal demonstrations brought together a variety of groups that included unionists, students and social groups.