A massive power failure blacked out Brazil’s two largest cities and other parts of Latin America’s biggest nation for more than two hours late Tuesday, leaving millions of people in the dark after a huge hydroelectric dam suddenly went offline. All of neighbouring Paraguay also lost power, but for only about 20 minutes.

“The huge Itaipu dam straddling the two nations’ border stopped producing 17,000 megawatts of power, resulting in outages in Rio de Janeiro, Sao Paulo and at least several other big Brazilian cities,” Brazilian Mines and Energy Minister Edison Lobao said. He said outages hit nine of the 27 states in a country of more than 190 million people.

The cause of the failure had not been determined, but Lobao said strong storms uprooted trees near the Itaipu dam just before it went offline and could be to blame. Rio was the hardest hit city, he said.

At 12:37 a.m. Wednesday, the lights in Rio’s Copacabana neighborhood flashed back to life, prompting cheers and thunderous car honking.

“It’s sad to see such a beautiful city with such a precarious infrastructure,” said Igor Fernandes, a 22-year-old law student peddling his bike down a dark Copacabana beach. “This shouldn’t happen in a city that is going to host the Olympic Games.”

Lobao 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 of the energy produced by the dam, and Paraguay consumes the rest.

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, the country’s leading newspaper, ABC Color, reported.

The company in charge of the dam, Itaipu Binacional, said the blackout did not start at the hydro electric complex. It said the most likely cause was a failure at one or more points in the transmission system.

The blackouts came three days after CBS’s “60 Minutes” news program reported that several past Brazilian power outages were caused by 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), snarling streets in Rio, where traffic that is normally chaotic turned riotous. Cars, taxis and buses zoomed through dark intersections, honking to let 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.

Flavia Alvin, 37, a shopkeeper in Copacabana, waited with her co—workers for the blackout to end before making the long bus ride home to western Rio. Asked if she was worried about violence or looting, she shook her head and pulled her young daughter closer.

“I’ve heard of problems like rioting in other places with blackouts, but Brazilians are more relaxed,” she said. “All I can do is wait here and drink a beer.”

That was what a crowd was doing at the Eclipse restaurant, a block from Copacabana beach. Drinking quickly warming beer at a restaurant beat sitting in a sweltering apartment, said Paulo Viera, 35, a graphic designer. But 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.

“The image of Brazil, of Rio, is bad enough with all the violence,” he said. “We don’t need this to happen. I don’t know how it could get worse.”

Subway service was knocked out in both Rio and Sao Paulo, and the G1 Web site of Brazil’s Globo TV said Sao Paulo subway users were forced to abandon train cars.

Some landing lights on runways at airports in Rio de Janeiro and Sao Paulo went dark, affecting take—offs and landings, according to Globo TV.

In the city of Taguatinga near the national capital of Brasilia, a second division Brazilian league soccer game was halted after lights illuminating the field went dark. No power outages happened in Brasilia.

Utility companies that provide electricity for Rio and Sao Paulo did not immediately offer explanations for why the power went off or when it would be restored, Agencia Brasil said.

Sao Paulo is South America’s largest city, with 12 million residents. Rio has 6 million citizens. But the metropolitan area of both cities are much larger. Also affected was Belo Horizonte in central Brazil and the North-eastern city of Recife.

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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