SAO PAULO (AFP) – A massive blackout across the southern half of Brazil has plunged tens of millions of people into darkness and prompted a major police mobilization amid fears of an opportunistic crime wave.

The country's largest cities, including Sao Paulo and Rio de Janeiro among others, were left with no illumination or traffic lights due to the outage late Tuesday, which the energy ministry said was caused by an undetermined problem at the country's biggest hydroelectric plant, Itaipu, on the border with Paraguay.

One radio station, Bandnews, said an estimated 50 million people -- one quarter of Brazil's population -- were affected.

The blackout hit at 10:15 pm (0015 GMT Wednesday).

The southern states of Sao Paulo, Rio de Janeiro, Minas Gerais, Espirito Santo, Mato Grosso do Sul and parts of the central state of Goias and the federal district of Brasilia were plunged into night.

Police in Sao Paulo and Rio called on the cities' residents to not go out into the darkened streets to avoid the risk of accidents and an upsurge in already prolific street crime.

Off-duty and vacationing officers were told to report to their posts.

In Sao Paulo and its suburbs, an agglomeration of 20 million inhabitants, streets were illuminated only by the lights of cars and from a few buildings -- including hospitals -- that had their own generators.

Traffic lights were extinguished, causing most motorists to nose carefully through intersections.

Taxis, normally numerous, were hard to flag down by stranded residents walking the suddenly darkened streets. Some of the drivers said they were wary of armed robbers taking advantage of the emergency.

Along Sao Paulo's main Avenue Paulista police and traffic wardens were deployed to ensure security and manage cars.

The Brazilian news website Abril said the new blackout occurred because 17,000 megawatts -- the amount required to power the city of Sao Paulo -- suddenly dropped out of the national electricity grid, according to the National Electric System Operator.

Energy Minister Edson Lobo confirmed that the problem originated with the Itaipu plant, whose output is shared with Paraguay.

There was a "complete paralysis" of that facility, the exact cause of which was still unknown.

Lobo said "atmospheric problems" could be to blame, perhaps a high-altitude storm with lightning that hit one of the plant's five high-tension lines.

The hydroelectric plant had been restarted, the minister said.

The head of Itaipu, Jorge Sanek, told Globo news that there was "no generation problem, the problem was with the transmission" of electricity.

The blackout occurred two days after US network CBS aired a report claiming massive power outages in Brazil in 2005 and 2007 were caused by cyber hackers attacking control systems.

Although Brazilian media were skeptical of that assessment, the US channel said those incidents should serve as a wake-up call to the United States, which a former Chief of US National Intelligence, Mike McConnell, warned could be about to take place.