Police used horses, pepper-spray and kettling tactics to clamp down Friday on an annual protest that has a history of getting rowdy.
They began searching bags and arresting people even before the start of Montreal's now-traditional springtime march against police brutality.
The march was swiftly declared illegal, and police worked to split the crowd into smaller groups and steer them off into side streets. Then came the kettling, the controversial law-enforcement tactic of surrounding a crowd and keeping people from moving.
There were some injuries, two of them to police officers. One cracked his teeth during the melee; another officer, feeling faint, was also taken to hospital.
Demonstrators have gathered in Montreal for the last 17 years to protest against police -- and 15 of those marches have seen violence.
This year's demonstration carried a uniquely bitter undertone after police and protesters clashed almost nightly during Quebec's so-called Maple Spring.
The events of last year remain hotly debated here, with many protesters arguing the worst violence at the student marches was committed by police -- not the demonstrators.
It appeared police had a zero-tolerance policy, right from the start of Friday's march. The event began front of Montreal police headquarters in the heart of downtown.
It was a stone's throw from the city's main shopping district and merchants had complained in recent days how business drops as people clear out of the area before the march.
Tempers flared, as they often have during the city's numerous recent protests. There were reports on social media of angry motorists inching their cars up against the street-clogging protesters.
Police advised people to avoid the area.
The march is almost automatically declared illegal because demonstrators don't provide a route to authorities, which is required under municipal bylaws.
Last year, 200 people were arrested after the march, which co-incided with ongoing student protests in the province, turned violent.
Windows were smashed in the melee and police cars were vandalized.
Police this year were co-ordinating their response to the march with a high-tech command centre that links them with the fire department and the city's ambulance service.
A large force of officers in riot gear was also deployed.
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