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A major electricity outage left tens of millions of people in Brazil's two biggest cities without power on Tuesday night due to problems with the transmission lines that connect to the massive Itaipu dam.
The blackout affected Sao Paulo and Rio de Janeiro, as well as cities in at least four other states, hitting the industrialized southeastern part of Latin America's largest country especially hard.
All of Paraguay, which gets most of its energy from the dam, was also briefly left in the dark.
Three hours after the blackout, power was beginning to be restored in some parts of Sao Paulo, Brazil's financial capital and South America's largest city. But much of the sprawling metropolis remained in the dark.
Traffic on the streets of Sao Paulo descended into chaos shortly after the power outage. Thousands of passengers were forced to exit stalled subway trains and walk along the tracks to get back to stations and make their way to the surface.
There were similar scenes in Rio, a tourist hub famous for its beaches and Carnival that is due to host the World Cup soccer championship in 2014 and the Olympic Games in 2016.
While initial reports pointed to problems at the Itaipu dam that straddles the border of Brazil and Paraguay, the dam's operator later said the blackout was probably caused by issues with the transmission grid.
"The most likely hypothesis is that there was some sort of accident that affected one or more points in the transmission system," the operator said in a statement. "This accident probably provoked others, a phenomenon that is known as the domino effect."
The generators at the dam -- the world's second-largest hydroelectric plant -- continued to run during the blackout but were unable to transmit any power to the grid, it said.
Brazil's energy minister, Edison Lobao, told reporters in the capital Brasilia that the exact cause was not yet known but "atmospheric problems, an intense storm, may have contributed to or caused the transmission lines to Itaipu to shut down."
Extra Police
Officials said they hoped power would be fully restored in the coming hours. The Itaipu power plant provides about 20 percent of the electricity supply in Brazil but more than 90 percent of Paraguay's.
At Rio's international airport, flights continued to take off and land but many passengers faced delays because taxi drivers were afraid to drive in the dark in crime-ridden areas.
No flight delays or cancellations were reported at Sao Paulo's international airport, which was operating on emergency generators.
CBN Radio reported that Rio's state governor, Sergio Cabral, had ordered extra police onto the streets.
Other Brazilian cities that suffered power outages included Belo Horizonte in the state of Minas Gerais and Campinas, a large city about an hour outside of Sao Paulo.
Brazil's national electricity grid operator said 17,000 megawatts of energy had been lost, equivalent to the entire consumption of Sao Paulo state.
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