Electricity was restored quickly, but the failure of the South American country's main power grid was another test for conservative President Sebastian Pinera, who took office last week and must rebuild after the huge earthquake.

The power cut rattled the nerves of Chileans still living with aftershocks following the deadly 8.8-magnitude quake on February 27 that triggered tsunamis and tore up roads and towns, causing an estimated $20 billion (13.2 billion pounds) to $30 billion in damage.

The lights went out at 8:50 p.m./12:50 a.m. British time, affecting millions of people and most of Chile's main cities in a stretch of territory more than 1,200 miles (2,000 km) long from north of the capital Santiago, to Puerto Montt in the south.

The quake-devastated city of Concepcion was among those plunged into darkness and some of the country's key copper mines briefly halted production.

Soon after midnight, power had returned to around 90 percent of Chile, a long, narrow country that produces copper, fruit and wine and is one of the most developed in the region.

SCARED IN THE SUBWAY

Claudia Morales, a 32-year-old nanny, was on the Santiago subway when the power cut hit, stranding commuters in the dark for around 15 minutes. Phones were down too, so they could not call anyone.

"Everyone started to say aloud maybe there had been another quake," she said. "Everyone was really panicked."

Passengers had to be evacuated from several subway cars, officials said.

Interior Minister Rodrigo Hinzpeter said the blackout was due to a problem with a-500 kilowatt transformer.

"Fortunately the reason for the failure has been identified," he said.

Hinzpeter said that while the power outage was not directly related to the big earthquake last month -- which knocked out power at the time -- there would be an investigation to see whether there was an indirect cause.

Energy Minister Ricardo Rainieri said the quake left Chile's power grid fragile, and urged ordinary Chileans to limit energy consumption.

MINES BRIEFLY SHUT

State mining giant Codelco, the world's top copper producer, said the disruption briefly shut its mines in central Chile. Its Teniente division operated with emergency equipment and output was partially affected, a mine official said, though power was returning to normal.

But Codelco's Norte Division in the northern part of the country was unaffected.

Global miner BHP Billiton's Escondida deposit, the world's largest copper mine, was operating normally and was not affected, a union leader said.

The Central Interconnected System, or SIC grid, was the one affected. It delivers power to many cities, including the capital.

But most of Chile's copper mines are on the northern power grid, so they were not affected.

Some Chileans took the power outage in stride.

"To be honest, I'm not scared," said 24-year-old mechanic Francisco Silva. "They say it was a power cut because the earthquake left the electricity lines weakened."

"I am calm, because I think the government can solve the problems," he added. "We have no option but to trust in it."

(Additional reporting by Alonso Soto, Fabian Cambero and Alejandro Lifschitz; Writing by Fiona Ortiz; editing by Simon Gardner and Stacey Joyce)