Protesters lobbed objects at officers, vandalized some stores and smashed police vehicle windows and flipped at least one over on to its roof. Authorities responded by firing off chemical irritants in a bid to disperse a crowd of about 1,000 people.
Late Thursday, Montreal police said they'd made more than 150 arrests.
An exact number of arrests and further information would be provided during a news conference on Friday, said police spokesman Ian Lafreniere.
Lafreniere said two police officers were injured lightly but he did not have any reports of protesters being treated at the scene.
At least two citizens were also slightly injured. Lafreniere said that one of them appears to have been a store owner who got into an altercation with protesters trying to protect his business from being vandalized.
About five hours after it began, police reported there were no longer pockets of protesters roaming the downtown area.
There was a surreal backdrop to some of the unruly scenes.
On the one hand, anarchists and anti-authority types tossed objects like garbage cans at police cars, and police responded by pumping chemical irritants into the air. In the background, regular crowds went about their ordinary business like evening shopping and eating out.
The march did disrupt some economic activity, however. Security at some Sainte-Catherine Street boutiques locked their customers inside to protect them. There were reports that some protesters helped themselves to video game consoles at a downtown Future Shop outlet.
"Smoke bombs aren't great for business," joked one store manager, yelling across the street at another one.
Police asked the protesters to disperse, but they merely headed north to the area not far from the Bell Centre. A large group was eventually surrounded by police near the city's central metro station, Berri-UQAM, where the lion's share of arrests were made.
Police reported some minor injuries like an officer being struck in the face with a rock and another hurt in a foot chase. Protesters had stories of their own, including one man who had a stun bomb explode just inches from his head.
"Yes some protesters are young and they lack wisdom, but that doesn't mean you toss stun grenades at them," said a man who goes by the name Number Juan, sporting a cut and burn on his forehead after a stun grenade went off near it.
He said he narrowly escaped serious injury because he was wearing ski goggles that were damaged in the blast.
"Let me say it was an attempted murder in my books and I'll be filing a complaint," he said.
Some protesters had been carrying around rocks, looking for targets. One police spokesman joked, in reference to the diverse, unpredictable crowd: "The problem with this type of protest is some people have a peace sign in one hand, and a rock in the other."
This year's anti-police march comes at a particularly sensitive time.
There have been battles in recent weeks at massive student protest marches against tuition hikes. One student suffered a serious eye injury amid a police intervention at a march last week.
Also heightening tensions are several shootings involving Montreal police, including one where an innocent bystander was killed by a stray bullet on his way to work at a hospital last June.
Police say 13 of the previous 15 editions of the march have degenerated into confrontations where arrests have been made.
Police had warned protesters a day earlier that would be expected to depart when police felt the protest had gone on long enough.
Protesters had a list of grievances for which they were marching including the memory of victims of police-related deaths and social and racial profiling.
A group called the Collective Opposed to Police Brutality, which organizes the annual event, never provides authorities with the protest route in advance as more mainstream protest organizers do.
"The route of the demonstration has not been provided and there exists no provision that obliges anyone to ask for a permit to demonstrate," the collective said in a statement.
They declined further interviews.
Organizers say other protests were planned in Toronto, Vancouver, Dallas, Brussels and Paris.Suggest a correction