Officials said the power failures began in the early morning and quickly spread across eight states in the vast region. An estimate by energy companies indicated at least 13 million people were affected.
In some areas, the power was only out for a few minutes, in other areas a few hours.
Mozart Bandeira Arnaud, director of operations at the Sao Francisco Hydroelectric Company, the biggest supplier of energy in the area, told the Globo TV network the power was mostly restored by 8 a.m. and that the problem originated in a substation that feeds high-transmission lines leading to three other energy suppliers, causing the failure to ripple across the region.
"There was a failure in an electronic component that was part of protection system of the substation," said Arnaud.
He said this triggered the security system of the Luiz Gonzaga substation in Pernambuco state to automatically shut down, cutting power to six high-transmission power lines running from the station, causing the blackouts to quickly spread.
"As the substation was very large, the effects were felt in several regions," Arnaud told Globo.
The blackouts hit four cities scheduled to host World Cup football matches in 2014, but Mines and Energy Minister Edison Lobao said officials were confident that energy supplies would not be a problem during the events.
He said and that there is a "committee established to work continuously through the Cup" to avoid such failures, though he did not describe what the committee was doing.
Lobao, speaking at a news conference in Brasilia, said a failure at the substation was the probable cause of the blackout, but that he and other officials would meet next week and provide a detailed report within days.
"We'll assess the damage and prevent this from happening again," said Lobao.
The minister defended Brazil's grid as "robust and modern" and said that such failures can happen in any nation. "There are flaws," he said of Brazil's energy grid, "but there are flaws in systems all over the world."
Energy expert Jose Jardini, a professor of electrical engineering at the University of Sao Paulo, said the failure was likely an "abnormality, an accident" and that it didn't seem to point to more severe problems, such as supply not meeting energy demand, since the blackout happened in the early morning hours when demand is low.
"The architecture of Brazil's electric system is robust, it's quite strong," he said. "This was an irregular event and we'll have to see what the investigation tells us. It shouldn't happen, of course, but it also seems to be an isolated event."
Dilton da Conti Oliveira, president of the Sao Francisco Hydroelectric Company, told reporters in Recife, a major city in the northeast where the company is headquartered, that "the most important thing now is that the cause of the problem is not preventing the restoration of energy."
Rio Olympic Committee officials, while not commenting on Friday's failure, said there are plans to ensure Rio's energy supply during the 2016 Olympics by creating an "energy island" and isolating the city from the nation's grid. The committee has not provided details on what that would entail and some experts have questioned how much it would safeguard Rio.
In 2009, a power failure left more than 60 million people in the dark in 18 states. Hard-hit then were Sao Paulo, South America's largest city, and Rio de Janeiro. Jardini said officials have yet to provide a substantial explanation for that failure, though they said at the time it was caused by a violent storm knocking down lines.
The worst of the recent Brazilian blackouts occurred in 1999 after lightning struck a power substation in Sao Paulo state, plunging 97 million Brazilians into darkness for up to five hours.
After severe energy shortages and rationing in 2001, Brazil began to improve the infrastructure of its power grid and diversify its energy supply.
During the two terms of President Luiz Inacio Lula da Silva, the government invested more than $13 billion on improving transmission lines and $5 billion more on transformers.
Much of the early planning in improving the power grid was carried out by now-President Dilma Rousseff, who was Silva's first energy minister.