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.
Do you have eyewitness video or photos of this incident? Please email them to email@example.com