Government and university buildings in downtown Toronto have been closed. Police said they had received reports of a suspicious character in the area, thought to be armed with a rifle.
Security officials at the university closed several buildings across the campus on Monday, after reports of an individual wearing a mask and possibly carrying a rifle.
Several units, including Toronto's tactical Emergency Task Force, were said to be at the scene.
The university published a list of buildings to avoid across the campus, which surrounds Ontario's provincial legislature.
"Please do not enter these buildings," said the university in a statement. "If you are in these buildings, please remain in place. Do not leave until an all-clear is called."
Police Chief Mark Saunders said Monday that they received a call about the man, and were later told that he might have a gun.
"It's clear to say that we live in a different time right now," said Saunders when asked if the public should be particularly vigilant after a nightclub shooting in Orlando, Florida, that killed 49. The incident, subsequently claimed by the so-called "Islamic State," claimed more lives than any past mass-shooting in the US.
Toronto Constable Allyson Douglas-Cook said the reports came from a building that was currently under construction. Douglas-Cook said the building security didn't recognize the individual, who was wearing a mask.