The utility said Montreal and other parts of southern Quebec were affected.
By 7:30 p.m. most customers had their power restored, and Hydro-Québec tweeted that its three major transmission lines shut down by heat and smoke from forest fires in northern Quebec were operational once again.
The blackouts occurred nearly exactly 24 hours after a similar power failure caused by the forest fires, despite the utility's best efforts to protect the network.
"The re-establishment of the lines was fairly fast," said Quebec Natural Resources Minister Martine Ouellet in an interview on CBC's French-language news network RDI.
"The heat, the smoke, the particles [from the fire] all interacted with the hydro lines, causing ionization," Ouellet explained. "That tripped the lines' safety mechanism."
"It's a good thing, as that protected the lines."
She said Hydro-Québec informed her it is spraying other equipment with foam to protect it from the heat of the fires.
Blackouts disrupt rush-hour traffic
The latest blackouts hit several municipalities in Greater Montreal and struck as far away as the Quebec City suburb of Ste-Foy, as well as parts of the Laurentians and the Lanaudière region.
Among the communities affected:
- Montreal's West Island, including the Fairview shopping centre.
- Montreal West.
- Lower Westmount, including the Alexis-Nihon plaza.
- Parts of downtown Montreal.
- St-Hubert in Longueuil.
The blackouts began at almost precisely the same time as they did yesterday, during the commuter rush hour, affecting traffic lights at many intersections and the lane designation lights on the Champlain Bridge.
Earlier today, Hydro-Québec's chief executive officer Thierry Vandal said the utility had taken steps to try to ensure there would not be a repeat of yesterday's outage, which shut down Montreal's Metro system and affected hospitals and shopping centres.
Wednesday's outage forced the Notre-Dame pavilion of University of Montreal's hospital network (CHUM) to cancel operations and left people trapped on stalled roller-coasters and other rides at LaRonde, the amusement park on Montreal's Île Ste-Hélène.
At Bell Centre, show goes on
Power was also out in some parts of the Bell Centre, where One Direction was set to play tonight.
Fans lined up around the block all day in anticipation of the boy band's performance.
At 6:20 p.m., promoter Evenko tweeted the news that power had been restored, and the concert would go on as planned.