The black-out began on Wednesday evening, and affected almost the entire city, Pakistan's biggest with more than 16 million people, and the country's commercial hub, a spokesman from the Karachi Electric Supply Co said.

'We're working to repair the fault and have restored electricity to some areas, but a big part of the city still remains without power,' the spokesman said.

'Hopefully things would normalise over the next few hours.'

An official from the Pakistan Electric Power Co, the country's main power body, said the fault occured after its lines were hit by a thunderstorm.

'Some other towns of Sindh were also affected by the disruption,' the official said, referring to the southern province of which Karachi is capital.

The Karachi Stock Exchange, which has its own power supply, was due to open at 9.30am (0330 GMT, 11.30am Singapore time)).

Spokesmen for Karachi's port, Pakistan's main port, were not immediately available for comment.

Karachi, like the whole country, has long faced electricity supply problems.

Residents, frustrated by prolonged black-outs, often vent their anger by blocking roads, burning tyres and throwing stones at police.

Karachi is known as Pakistan's 'city of lights', but many residents spent a sleepless night in summer heat with no fans or air conditioning. Essential services were suspended, and major industries were shut.

Pakistan is experiencing a major electricity crisis, with many people facing up to 12-hour cutoffs due to soaring demand and a lack of investment in generating capacity. -- REUTERS, AP