But no threat was detected and no one was injured, police say. The whole thing was a hoax, where someone inside the building made the phony call to the Montreal police.
The Montreal police are now evacuating the building after identifying no threats.
Meet Patel, 24, who lives near the Ubisoft office, was hanging out with a friend when they saw police rapidly begin congregating around the building. The bad news is, the whole thing was a hoax, and dozens of people were terrified for hours, stranded on a roof.
The operation involved a heavy use of police resources, aside from other disruptions, including the "code orange" preparation put into place at a downtown hospital - a very rare warning used in cases where there may be mass casualties. An unverified tweet from Yara El Soueidi, writer for Cult MTL, claimed that a friend of hers that is an Ubisoft employee claimed their co-workers are now hiding under their desks.
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Montreal Mayor Valerie Plante told locals to avoid the Mile End suburb.
A member of Ubisoft working in production marketing said on Twitter "I got out and am okay".
Ubisoft has multiple buildings in the area.
"Police officers were dispatched to the scene to respond to a 911 call", a police spokesperson said, according to AFP. The tweet also stated that they are "currently validating information" and will provide details as they come.
Hugo Lefevre, who works at Ubisoft, said he had just returned from lunch when a rumour started spreading through the building that there was a potential hostage situation.