Live television coverage showed first responders performing CPR on the man before placing him in an ambulance.
A police spokesman said the man's life is in danger and a number of officers were also being treated for shock.
He also confirmed that police are looking for a witness who they believe may have been attacked by the man, who was wielding a hammer.
Another witness told CBC News that the man had broken windows at a nearby hotel before he was confronted by police.
Police have closed Berri Street between Ontario Street and de Maisonneuve Boulevard East.
More details to follow.
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