The power outage Tuesday night affected 40 million people and left about 800 cities - including Sao Paulo, Rio de Janeiro, Belo Horizonte and Brasilia - in the dark.
Energy Minister Edison Lobao speculated that a storm had disconnected power transmission lines at the Itaipu dam, which provides more than 20 percent of Brazil's energy.
Technicians were working to get the power supply back on line quickly, both Lobao and Itaipu's operating company said. Power is being restored in some areas of the city, more than two hours after it went out.
The power outage began shortly after 10 p.m. (midnight GMT) and was starting to be restored early Wednesday.
In the meantime, thousands of people were trapped in elevators, subways and suburban trains. Road traffic was also chaotic as the power outage darkened traffic lights, and the police were put on high alert as an outbreak of crime was feared.
Only large office buildings and hotels with generators were lit, DPA correspondents said.
The country's largest airports were also using generators and providing limited emergency service, they said.
Brazil's phone network largely collapsed, but its mobile-phone network was still operating.
Neighbouring Paraguay, for which Itaipu also provides power, said a short circuit in a transmission line near Sao Paulo set off a chain reaction, taking Itaipu automatically off the network and causing the blackout.
Large parts of Paraguay also went dark, but the blackout there lasted only 20 minutes.
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