Lula seeks explanation for huge Brazil blackout

* Power restored after huge blackout hits Sao Paulo, Rio

* President Lula summons energy minister for explanation

By Eduardo Simoes

SAO PAULO, Nov 11 (Reuters) - Brazil's president sought an urgent explanation on Wednesday for the worst power outage in a decade, which left a huge swath of the country in the dark for more than five hours and raised doubts about the reliability of its energy infrastructure.

The blackout on Tuesday night left tens of millions of people without power across most of the country's wealthy southeastern region, halting subways and snarling traffic in major cities like Sao Paulo and Rio de Janeiro.

The cause of the outage was unclear. Energy officials said the giant Itaipu hydroelectric dam had shut down, but the Itaipu Binacional company that runs the project said in a statement on Wednesday that problem originated elsewhere.

It said the dam on the border between Brazil and Paraguay had been functioning normally but had not been able to transmit energy because power lines were not working properly.

"We haven't established the cause of the problem yet," Energy Minister Edison Lobao told the O Globo news network.

Lobao earlier told reporters that a storm may have caused power lines from Itaipu to shut down, causing a chain reaction that cut service throughout Brazil and Paraguay, which gets about 90 percent of its electricity from the dam.