Power was gradually being restored to major cities across Pakistan Sunday after it was hit by a massive electricity blackout, officials said.

The electricity distribution system in the nation of more than 210 million people is a complex and delicate web, and a problem in one section of the grid can lead to cascading breakdowns countrywide.

The latest blackout was caused by "an engineering fault" in southern Pakistan at 11:41 pm local time on Saturday (1841 GMT), which tripped the system and caused power plants to shut down, power minister Omar Ayub Khan told a press conference in Islamabad.

-Our experts are trying to determine the exact location of the fault.-

Khan said that will take "another few hours as the area is still covered in dense fog", but that power had been partially restored in most areas of Punjab, the most populous province, as well as the economic hub Karachi in the south.

"We hope to bring the system back to its full capacity by this evening, but it will take some time for nuclear and thermal power plants to get operational," Khan tweeted.

People were cracking jokes and exchanging memes on Twitter, Facebook and WhatsApp, mostly ridiculing Prime Minister Imran Khan's government and its performance after the breakdown.

"Power breakdown in Pakistan is blackmailing Imran Khan," tweeted Musarrat Ahmedzeb in reference to the premier's recent statement accusing Shiite protesters of blackmailing him after killing of 10 miners.

"What a start for the new year... let us seek Allah the Almighty's mercy," read another tweet, while a message on WhatsApp said: "new Pakistan sleeps in a night mode".

There were no immediate reports of disruption at hospitals, which often rely on back-up generators.

Netblocks, which monitors internet outages, said web connectivity in the country "collapsed" as a result of the blackout.

Connectivity was at "62 percent of ordinary levels", it said in a tweet.

This was Pakistan's second major power breakdown in less than three years. In May 2018, power was partially disrupted for more than nine hours.

In 2015, an apparent rebel attack on a key power line plunged around 80 percent of Pakistan into darkness.

That blackout, one of the worst in Pakistan's history, caused electricity to be cut in major cities nationwide, including Islamabad, and even affected one of the country's international airports.