The Itaipu dam straddles the border of Paraguay and Brazil. Outages were reported in Rio de Janeiro, Sao Paulo and at several other big Brazilian cities.
Brazilian Mines and Energy Minister Edison Lobao said that the hydroelectric plant at the dam was working, but that there were problems with the power lines that carry electricity to the rest of Brazil. He noted that storms uprooted trees near the dam just before it went offline and could be to blame.
Millions of people were affected by the blackout, which affected six Brazilian states, as well as parts of Paraguay. While the cities affected included the cities of Rio de Janeiro and Sao Paulo, the capital, Brasilia, was unaffected.
The Itaipu dam is the world's second biggest hydroelectric power producer. It supplies 20 percent of Brazil's electricity. China's Three Gorges dam is the largest.