Flames are under control in a five-storey commercial building on Sparks Street, downtown.
The Ottawa Fire Service received multiple 911 calls reporting heavy black smoke from the roof at 93 Sparks St., just after 2 p.m.
No injuries have been reported, as the fire was deemed under control around 3:15 p.m.
Police said the blaze affected vehicle and pedestrian traffic on Sparks Street as well as Metcalfe Street, between Queen Street and Wellington Street.
The incident is still under investigation.