A blackout has hit a large part of Venezuela, darkening street lights, shutting down the Caracas underground and forcing President Hugo Chavez's government to resort to temporary rationing measures.
The power outage affected the capital of Caracas and 12 states stretching across the northern half of the country, electricity minister Ali Rodriguez said.
The state oil company said some activities were interrupted at its refineries, including units partially halted at the Jose heavy crude upgrading complex in eastern Venezuela.
But it said operations were expected to be back to normal by early on Friday.
A forest fire apparently caused the outage by overheating major transmission lines in western Venezuela and knocking them off-line, Mr Rodriguez said.
He said that failure forced various power plants to shut down and caused a cumulative drop of about 10,000 megawatts, a large share of the country's generating capacity of roughly 24,000 megawatts.
Mr Rodriguez told state television that power had been completely restored throughout the country about two hours after the outage began. He said it had been a power loss of "enormous size".
Vice president Elias Jaua vowed a thorough investigation of the causes of the forest fire. He said it was unclear how the fire started but that "we never stop investigating situations of possible sabotage".
He suggested sabotage would not be beyond some hard-line opponents of Mr Chavez, saying Venezuela has "extremist sectors that still don't accept the democratic game".
Various power plants were being gradually restarted after the blackout, Mr Rodriguez said.