A massive blackout hit a large section of Venezuela, crippling parts of the capital's subway system.
The power outage rocked nine of the country's 23 states have been affected by Thursday's power loss, Electricity Minister Ali Rodríguez.
He says major transmission lines failed in northwestern Venezuela, causing a drop of 6,000 megawatts.
Rodríguez tells state television that power is gradually being restored to affected areas of Caracas, the capital.
There are no immediate reports of problems in Venezuela's oil industry. The country's refineries are equipped with emergency generators.
The blackouts come just six months after President Hugo Chávez just months after electricity rationing hit his popularity.
State television said electricity was off in at least six states, with commuters flooding the streets of the capital after the metro ground to a halt at the start of the evening rush hour. Many traffic lights went out across the city.
The country's main refineries were operating normally, state oil company PDVSA and a union said.
"We are confident services will be re-established very quickly," Rodríguez said.
He said the outages were caused by the failure of an 800 KW cable, affecting 6,000 MW of capacity.
Just last week, outages swept across Venezuela, plunging a number of states into darkness and causing intermittent problems on the metro, which carries some 2 million people a day.
Throughout 2010, Venezuelans were subjected to strict electricity and water rationing blamed by the government on a drought caused by the El Niño weather phenomenon. Combined with an economic slump, the problems with utilities pushed Chávez's popularity ratings down.
Rainfall and heavy investment in new oil-fired power stations helped overcome the crisis ahead of parliamentary elections in September. But experts warned the national grid was still running close to capacity.
Chávez declared a national emergency last year but canceled a plan to ration electricity in Caracas after a chaotic first day of cuts left poor, crime-ridden neighborhoods in the dark and workers stuck in elevators.
The socialist president who was first elected in 1998 and draws his support largely from working class Venezuelans is preparing a re-election bid in December 2012.
Based on reporting by The Associated Press.