Hydro-Québec confirmed the power failure was related to the spreading forest fires around Eastmain in northern Quebec near James Bay.
Spokeswoman Marie-Hélène Devault said the intense heat and fire there caused heat-sensing electric breakers to shut down.
She said only industrial and commercial clients such as the STM, Montreal's transit authority, and La Ronde amusement park in Montreal were affected, and not residential customers.
Just before 7 p.m. on Wednesday, Hydro-Québec spokesman Patrice Lavoie said 1,873 clients were without electricity, with the bulk of them being in Quebec's northern communities.
He said 547 clients in Montreal had been affected as well.
Photos of people trapped on rides at La Ronde proliferated on Twitter, as did images of people standing on Metro platforms in the dark.
The service had resumed on all Metro lines by 6 p.m.
Devault said Hydro-Québec's equipment is in good condition and that measures were being taken to restore electricity.