(CNN) -- Power returned slowly to Venezuela on Tuesday night, a few hours after widespread outages blacked out nearly half the country, trapping people in elevators, stalling subways, filling streets with pedestrians and forcing hospitals to switch to emergency generators.
The streets of Caracas, Venezuela, are thronged with people during Tuesday's power outage.
"We understand that at 1600 hours -- that is to say at 4 p.m. (4:30 p.m. ET) -- there was a blackout on a national level that was produced by an explosion, which is being investigated" in the Guri hydroelectric power station, Caracas Mayor Juan Barreto said.
"That produced a blackout in 16 states of the country, particularly those of the central north coastal area."
The city's emergency plan kicked in, and about 400 firefighters in training had reported for duty to help, he said. City firefighters had gone to 37 buildings in the capital to free people from elevators.
The head of the country's electric authority, Hipolito Izquierdo, told state-run Venezuelan Television that 60 percent of the power had been restored by 7 p.m. local time (7:30 p.m. ET) and predicted that power would be fully restored before 9:30 p.m. (10 p.m. ET).
"We are now recovering," he said.
Minister of Interior and Justice Ramon Rodriguez Chacin blamed a fault in a generator at the plant in Guri, which caused a high-voltage power transmission line to overheat. In taking the line out of service, power was disrupted to other areas, he said.
In all, about 40 percent of the country was blacked out, officials said.
Affected zones included parts of the capital city of Caracas -- where streetlights were dark -- and the states of Zulia, Lara, Carabobo, Yaracuy, Portuguesa, Miranda, Falcon, Merida, Aragua, Bolivar and Tachira, the station said.
Gustavo Gonzalez, the president of Caracas Metro, the city's rapid-transit system, said it was functioning normally. But a ticket seller at one station said train service had been halted and riders were being diverted to buses.
"Normally, these blackouts don't occur very often," said Javier Alvarado, president of Caracas Electricity. E-mail to a friend