Airport operations were hindered and subways ground to a halt. All of neighboring Paraguay also went dark, but for less than a half hour. A spokesman at Brazil's Energy Ministry said up to 60 million people -- nearly a third of the nation's population -- were affected by the blackout. He spoke on condition of anonymity, as he was not authorized to discuss the matter.
Brazilian authorities blamed storms that took down power lines and towers, causing a domino effect that rippled across the region.
Lights twinkled back on along Rio's Copacabana beach, in South America's largest city of Sao Paulo and in Paraguay's sleepy capital of Asuncion. But some traffic lights were still out in both Rio and Sao Paulo and traffic officials were expecting drivers to face difficulties the rest of the day, according to local media.
In Rio, Governor Sergio Cabral sent an elite police unit into the streets early Wednesday to help maintain calm in a city known for its crime. The mayor dispatched 300 extra unarmed civil guards to help control traffic.
The city saw a spike in robberies around the Maracana football stadium -- where the opening and closing ceremonies of the 2016 Olympics will be held, along with the 2014 World Cup final. A police spokesman said a band of roughly a dozen criminals worked the area together, robbing people en masse -- a crime phenomenon so routine it is known as a "big sweep."
The spokesman, who talked on condition of anonymity as he was not authorized to discuss the matter, didn't say how many robberies occurred or how many happen in an average night in the area, but said there were more than usual during the blackout. He said elsewhere in Rio there did not appear to be much of an increase in crimes, which he in part attributed to commuters staying within the safety of train and subway stations until energy was restored.
Questions remained about what happened and what the fallout would be in Brazil, a nation seen as an ascending economic and political power.
"The image of Brazil, of Rio, is bad enough with all the violence," said 35-year-old graphic designer Paulo Viera, as he sat in a restaurant a block from the sandy arc of Copacabana.
Standing in an open-air restaurant where patrons were drinking quickly warming beer, Viera said he worried about how the outage might look for a city that last month was picked to host the 2016 Olympics and will be the showcase city for soccer's World Cup in 2014. "We don't need this to happen. I don't know how it could get worse."
The blackout comes on the heels of a wave of gang fighting in Rio's slums that led to violence fears for the Olympics.
"It's sad to see such a beautiful city with such a precarious infrastructure," said 22-year-old law student Igor Fernandes. "This shouldn't happen in a city that is going to host the Olympic Games."
The outage occurred when the huge Itaipu dam straddling the Brazilian-Paraguay border stopped producing 17,000 megawatts of power, Brazilian Mines and Energy Minister Edison Lobao said. He said outages hit nine of the 27 states in this country of more than 190 million people. No power outages happened in Brasilia, the national capital.
Jorge Miguel Samek, the head of Itaipu Binacional, the agency in charge of the dam, said there was a "99 percent chance the blackout happened because of a storm."
"There was no problem with generating electricity, but a problem with "lightning or a storm that took down some towers," he said.
In a Wednesday statement, the agency said that despite the fact it never stopped functioning, "there was no possibility of transmitting energy because the transmission lines that connect Itaipu to the Brazilian system were disconnected."
Lobao also said the hydro plant at the dam itself was working, but there were problems with the power lines that carry electricity across Brazil. Brazil uses almost all the energy produced by the dam, and Paraguay consumes the rest. About 80 percent of Brazil's energy comes from hydroelectric power.
In Paraguay, the national energy agency blamed the blackout on a short-circuit at an electrical station near Sao Paulo, saying that failure shut down the entire power grid supplied by Itaipu. All of Paraguay went dark for about 20 minutes, ABC Color newspaper reported.
The blackouts came two days after CBS's "60 Minutes" news program reported that several past Brazilian power outages were caused by computer hackers. Brazilian officials had played down the report before the latest outages, and Lobao did not mention it.
Brazil's official Agencia Brasil news agency said Tuesday's outage started about 10:20 p.m. (1220 GMT; 7:20 a.m. EDT), snarling streets in Rio, where traffic that is normally chaotic turned riotous.
Cars, taxis and buses sped through dark intersections, honking to make their presence known as they zoomed through. Pedestrians scampered across avenues, and tourists scurried back to a handful of luxury beach hotels, the only buildings with light.
The Itaipu dam is the world's second biggest hydroelectric producer, supplying 20 percent of Brazil's electricity. China's Three Gorges dam is the largest.
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