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 a fault in southern Pakistan at 11:41 pm local time on Saturday (1841 GMT), power minister Omar Ayub Khan tweeted, citing preliminary reports.

"The fault tripped the transmission system of the country... leading to the shutdown of power plants," Khan said.

The blackout plunged all of Pakistan's major cities into darkness, including the capital Islamabad, economic hub Karachi and the second-largest city Lahore.

Khan said "the breakdown took place when frequency tripped from 50 to zero in the electricity distribution system. However, electricity is gradually being restored across the country".

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

A water and power ministry spokesman said power had been restored to some parts of the country but many areas in Lahore and Karachi were still waiting.

He added that an investigation has been launched to probe the cause of the blackout.

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

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

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.

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