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Pakistan's Power Minister Omar Ayub wrote on twitter that power was restored to most of the country's capital Islamabad by Sunday morning, as well as other areas, including the city of Peshawar.

He later said that power was restored to most of the country.

The countrywide blackout was caused by a "sudden plunge in the frequency in the power transmission system" before midnight on Sunday, which caused power plants to shut down, according to Khan.

Government officials appealed for calm as local voiced concerns.

"There is a lot of fear" Usama Khan, a resident of Karachi, told Sky News. "Sometimes just one or two cities lose power but we can’t understand why all of Pakistan has lost power", he added.

Large areas of Karachi, Pakistan's largest city, did not have power by the early hours of Sunday morning, according to K-Electric, the company which supplies power to the city, which said that power was being restored to this city in phases.

"There are long lines outside petrol pumps in the city, cars are queuing as people buy fuel for their back up generators." Akbar Saifi, another Karachi resident, told CNN.

It is not clear what cause the sudden the drop in frequency at Guddu Thermal Power plan in Pakistan's southern Sindh province, but the power minister said that an investigation was underway.

Read also: Netblocks, an internet monitor which tracks shutdowns, said that connectivity had collapsed in the country due the outage.

However, PIA, Islamabad's flag-carrier, reported no disruption to its flights.

Power cuts are not unheard of in Pakistan, with essential facilities such as hospital often relying on diesel-fuelled generators as a back-up power supply. Some Pakistanis have back-up petrol generators at home in case of power cuts.

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