STORY: Hydro One, Ontario's biggest electricity distribution company, said in a tweet it was investigating reports that a barge carrying a crane came into contact with high-voltage transmission lines and it was trying to reroute power to Toronto Hydro, which distributes electricity in downtown Toronto.
Toronto Hydro said nearly 10,000 customers were affected.
Toronto is Canada's biggest city and its financial capital, and the central business district is home to the biggest banks, insurers and stock exchange operator.
The power failure saw workers from downtown towers spill onto the streets, taking to sidewalks and parks, while shutting down many cafes and restaurants during the peak business hours.
Toronto Fire Services said it had responded to "numerous calls" for people trapped in elevators during the power outage.