A blackout across much of southern Brazil overnight plunged tens of millions of residents in the country's biggest cities into darkness.
The two largest cities of Sao Paulo and Rio de Janeiro were both affected by the blackout.
The outage, which started at 10.15pm, originated at the Itaipu hydroelectric plant that straddles the border between Brazil and Paraguay, supplying both with much of their energy needs.
Brazilian Energy Minister Edson Lobao said it was not yet known what caused the unprecedented ‘complete paralysis’ of the plant, but speculated that lightning from a storm might have shorted one of the facility's five high-tension supply lines.
The plant provides around 20% of the energy needs of Brazil, Latin America's most populous and economically important nation.
Paraguay, which gets 90% of its electricity from the installation, was virtually without power for at least 15 minutes because of the problem, according to reports.
While Brazil has encountered severe power shortages in the past, notably in 2001, 2005 and 2007, the scale of the blackout was extraordinary.
One radio station, Bandnews, estimated that 50m million people, or more than a quarter of Brazil's 190m population, were suddenly thrown into the dark.
Fearing an opportunistic crime wave and accidents because of non-functioning traffic lights, police in Sao Paulo and Rio called up off-duty officers and urged the public to stay off the streets.
There were no immediate reports of disturbances.
