Pakistan has been plunged into darkness after the breakdown of a key power transmission line in the southern part of the country.

The power failure, one of the worst Pakistanis have experienced, has caused power to be cut in major cities throughout the country including the capital Islamabad.

The blackout began when a 500kV transmission line carrying electricity from the private sector Hubco Power Plant to the grid tripped.

The country's state minister for water and power apologised for the inconvenience and said work was underway to restore electricity.

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"On the Prime Minister's directive, we are not to sleep till this problem is resolved," he said in a message on Twitter.

A senior official at the National Grid station in Islamabad said it will take about eight hours to restore electricity to the country.

"Around 80 per cent of the country is under the spell of the power breakdown. It will take around eight hours to restore electricity," the official said on condition of anonymity.

A reporter in the eastern city of Lahore said the airport had also been affected by the power outage.

-AAP