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 anightclub 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.