A malfunction at one of Pakistan's main power stations set off a chain reaction that shut down other power stations across the country, leaving millions in darkness.
A major technical fault in Pakistan's power generation and distribution system caused a massive power outage that plunged the country into darkness overnight, the energy minister said.
The blackout affected all the country's major cities on Saturday, including its capital Islamabad, its major economic hub Karachi, and the second-largest city, Lahore.
Hours after the late outage, Energy Minister Omar Ayub Khan said on Twitter that power was being restored in phases, starting with Islamabad. He said later on Sunday that power had been brought back to much of the country.
The blackout was initially reported on social media by residents of major urban centres, including Karachi, Lahore, Islamabad and Multan. The minister and his spokesman then took to Twitter to update the country.
Khan urged people to be patient. He said the cause of the power outage was being investigated and work was being done to fire up Pakistan's main Tarbela power station in the northwest, which would lead to a restoration of power in the rest of the country.
Power cuts are common in Pakistan, which has been facing a chronic energy crisis for years and has a complex and outdated distribution system. Many of its approximately 210 million inhabitants are without power for long hours every day, and the phenomenon is aggravated during the hot summer months.
The Guddu power plant in southern Sindh province developed a fault at 11.41 pm local time (6.41 pm GMT) that triggered a chain reaction that shut down other power plants in seconds, Khan said in a news conference on Sunday.
"Our experts are trying to determine the exact location of this malfunction," he added, adding that it would take "a few more hours because the area is still covered by dense fog".
The blackout, one of the worst in Pakistan's history, affected one of the country's international airports and forced the country's hospitals to use their backup generators.
Power has been partially restored in most of Punjab, the most densely populated province in the country, as well as in Karachi, he said. This was also the case in Islamabad and Lahore in the late morning.
"We hope the system will be operating at full capacity again by this evening (Sunday), but it will take time before the nuclear and thermal power plants are fully operational," Khan said on Twitter.
Later, Zafar Yab, spokesman for the Ministry of Energy, said the Tarbela and Warsak plants, both in northwestern Khyber Pakhtunkhwa province, had come back online and power was being restored to the transmission system.
Yab said the restoration of power to all areas of the country would take some time, however.