The video, obtained by CBC News and only a few seconds long, shows an arrest already in progress as three officers are trying to subdue a man who is resisting.
In the video, the officers are holding the man's arms while another punches him several times, with a couple of punches landing around the man's face.
The officers then lift the man in the air and bring him down. People are heard screaming off-camera.
CBC News reported the man who shot the video, Clauderic St-Amand, said the protester looked as though he was separated from the larger anti-capitalist protest in downtown Montreal and was sitting on a street corner before police intervened.
Montreal police spokesman Ian Lafreniere told CBC on Monday the force is investigating and trying to identify the officers in the video.
Hundreds of protesters took to the streets Friday evening in downtown Montreal for the annual protests celebrating International Workers' Day, also known as May Day.
This year's protest turned violent with protesters and police clashing for hours in the city's downtown core.
Five people, including two police officers, were treated for minor injuries while 27 people were arrested and face various charges including illegally gathering and assaulting a police officer.
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