People in Rio de Janeiro used candles in the blackout
A power blackout in Brazil left tens of millions of people sitting by candlelight, after plunging its two largest cities into darkness.
Underground railways, traffic lights, street lamps, lifts and electric gates in Sao Paulo and Rio de Janeiro were all affected.
It was several hours before the problem could be fixed.
The authorities believe it could have been caused by a fault at the giant Itaipu hydro-electric dam.
AT THE SCENE
Gary Duffy, Sao Paulo
Walking around on the streets near my home, there was absolute chaos, with drivers pulling over to the side of the road and asking what had happened, and did anyone know what was wrong.
The traffic lights were out of order, traffic wardens were trying to maintain the flow of traffic, radio stations were appealing to people to drive carefully.
There was chaos in the metro system as well, some reports that people had to walk along the tracks because trains had come to a halt, that bus drivers were being called in to provide an emergency service.
The underground railway systems in Rio de Janeiro and Sao Paulo shut down when the power cuts hit soon after 2200 (0000 GMT), leaving many passengers stranded.
Thousands of rail passengers had to walk down underground tracks to reach stations.
No traffic lights or street lights were working, causing traffic to stop or slow to a crawl.
Extra police were put on the streets to prevent a surge in crime.
Electricity supplies in Paraguay, which shares power from Itaipu, were also disrupted for a short while.
The power system lost 17,000 megawatts after the massive plant went offline, possibly because of a storm.
The director of the dam said it had lost its entire hydro-electric output.
The BBC's correspondent in Sao Paulo, Gary Duffy, says the power cut happened at a time when millions would have been watching the country's popular soap opera on TV.
He adds that neighbourhood blackouts are common in the city of 19 million, but the scale of this power cut was remarkable.
The issue of power supplies is politically sensitive in Brazil following severe shortages several years ago which caused blackouts in large parts of the country.
The government will be keen to establish that this blackout, which disrupted the lives of millions of Brazilians, was not caused by any failure on its part, our correspondent says.
The latest power failure also affected the south-eastern states of Minas Gerais and Espirito Santo, the south-western state of Mato Grosso do Sul, parts of the central state of Goias, and the federal district of Brasilia, although the capital itself was unaffected.
Luckily I got out of the lift about a minute before everything shut down... [and] at least we managed to have a romantic candlelight supper.
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