A massive power failure blacked out Brazil’s two largest cities and other parts of Latin America’s biggest nation overnight, leaving millions of people in the dark after a huge hydroelectric dam suddenly went offline.

Paraguay was also affected when the 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.

The cause of the failure had not been determined, but the Brazilian Mines and Energy Minister said strong storms had uprooted trees near the Itaipu dam just before it went offline and could be to blame.

Rio de Janeiro, which is due to host the World Cup soccer championship in 2014 and the Olympic Games in 2016, was the hardest hit city. Rio’s famous Copacabana beach, usually illuminated by the bright lights from the skyscrapers lining the boulevard, was plunged into darkness.

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

Flavia Alvin, 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.

Quickly drinking warming beer while sitting in a sweltering apartment, a graphic designer Paulo Viera worried about how the outage might look for a city that last month was picked to host the Olympics.

“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.”

According to officials, 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.

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 Mr Lobao did not mention it.

The outage started about 10:20pm (0020 GMT), snarling streets in Rio after traffic lights stopped working. Subway service was knocked out in both Rio and Sao Paulo, and G1 said Sao Paulo subway users were forced to abandon train cars, leaving thousands of people stranded.

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.

Shortly after midnight local time, more than two hours after the blackout occurred, lights began to flicker back on in Brazil’s major cities.

In Rio’s Copacabana neighbourhood, cars honked wildly and people shouted as power was restored to the area.

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.

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 northeastern 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.