Several other cities were also affected by the power failures which lasted more than two hours.
7,000 megawatts of power after the massive dam went offline, possibly because of a storm.
The director of the dam said that it lost its entire hydroelectric（水力电气的） output, but power is now largely restored.
Neighbouring Paraguay also suffered a brief, total power failure.
In Brazil's major cities, the underground railway systems in Rio de Janeiro and Sao Paulo shut down, after the power cuts hit soon after 2200 (0000GMT), leaving many passengers stranded（搁浅的）.
Thousands of rail passengers had to walk down underground tracks to reach stations.
No traffic lights or street lights were working in Rio, the hardest hit city, and Sao Paulo, forcing traffic to stop or slow to a crawl.
Crime concerns
Extra police had been ordered onto Rio's streets to prevent any opportunistic（机会主义的，投机取巧的） crime, reports said.
The BBC's correspondent in Sao Paulo, Gary Duffy, said the power outage happened at a time when millions would have been watching the country's popular soap opera on TV.
Neighbourhood blackouts are common in the city of 19 million, but the scale of this outage was remarkable, he added.
The power failure also affected the southeastern states of Minas Gerais and Espirito Santo, the southwestern state of Mato Grosso do Sul, parts of the central state of Goias, and the federal district of Brasilia, although the capital itself was unaffected.
In all, nine of Brazil's 27 states were affected.
The Itaipu dam provides Brazil with 20% of its electricity.
Mines and Energy Minister Edison Lobao said a large storm in the area of the dam might have caused the power failure.
"The exact cause still isn't known, but we suspect that atmospheric（大气的） problems, an intense storm, may have contributed to or caused the transmission lines to Itaipu to shut down," he said.
In Paraguay, which relies on the Itaipu dam for 90% of its electricity, the entire country went black for 15 minutes, but reports from the capital Asuncion say that power has now returned.
