The blackout began at about 11.30 pm after a privately run 1,200 MW thermal power plant in southwestern Balochistan province developed a fault and stopped generating electricity.
"The HUBCO (Hub Power Company) plant developed a fault and the load was transferred to the Mangla and Tarbela hydel projects but they tripped. After that, several smaller powerhouses tripped," Javed Parvez, CEO of the Islamabad Electric Supply Company, told the media.
The Mangla and Tarbela power projects had resumed generating electricity and efforts were being made to restore the supply of power to cities in phases, Parvez said.
The Prime Minister was monitoring efforts to restore power supply. Parvez declined to say how long it would take to bring the entire national grid back on line.
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TV news channels reported that almost 80 per cent of the country was without power.
The blackout affected operations at several airports and railway services.
Reports said most cities in all four provinces were without power, including Peshawar, the capital of northwestern Khyber-Pakhtunkhwa, and Quetta, the capital of southwestern Balochistan.
Reports said more than 30 grid stations had tripped in Karachi, the country's largest city. In Balochistan, at least 18 districts were affected by the blackout.