Const. Ian MacDonald says several 911 callers reported a distraught man yelling and brandishing a large butcher-style knife outside a building on Wednesday afternoon.
He says officers attempted to communicate with the 57-year-old man, but he wouldn't comply with their demands.
MacDonald says police subdued him by firing plastic bullets that don't pierce the skin.
The man has been taken to hospital for treatment and is expected to get a mental-health assessment.
Police are asking witnesses to forward any photos or video of the incident.