A massive power cut across Pakistan continued after night fell on Monday, affecting most of the country's 220 million residents, including in the metropolises of Karachi and Lahore.

Pakistan's power system is a complex and delicate web, where problems can quickly cascade.

The breakdown was caused by a fault in the national grid at about 7:30 am (0230 GMT), linked to a cost-cutting measure as the country's economy ails.

"We hope that the electricity will be restored throughout the country by tonight," Energy Minister Khurram Dastgir Khan said in a video statement.

A variation in frequency on the national grid caused the cut, as power generation units were turned on early in the morning.

The units had been temporarily switched off at night to save fuel, Khan earlier told the media.

Localised power cuts are common in Pakistan and hospitals, factories and government institutions are often kept running by private generators. The machines are, however, beyond the means of most citizens and small businesses.

In parts of northern Pakistan, temperatures were due to drop below freezing on Monday night with supplies of natural gas -- the most common heating method -- also unreliable due to load-shedding.

The economy is already hobbled by rampant inflation, a falling rupee, and severely low forex reserves, with the power cut piling extra pressure on small businesses.

In the garrison city of Rawalpindi, homeware trader Muhammad Iftikhar Sheikh, 71, said he was unable to demonstrate electronic products to browsing customers.

"The customers never buy without testing first," he said. "All of us are sitting idle."

Schools mostly continued either in the dark or using battery-powered lighting.

A shop owner in the southern port city Karachi, where temperatures were higher, told AFP he feared his entire dairy stock would spoil without refrigeration.

Printer Khurrum Khan, 39, said orders were piling up because of the blackout.

Unreliable power is "a permanent curse which our governments have failed to overcome", he complained.

Repair work was under way, with limited power restored in some parts of the country, including the capital Islamabad, and the western city of Quetta.

Karachi, with a population of more than 15 million, and Lahore, home to more than 10 million, both remained largely without power as darkness fell.

Mobile phone services were also disrupted as a result of the outage, the Pakistan Telecommunication Authority tweeted.

A similar breakdown in January 2021 affected the entire country, after a fault occurred in southern Pakistan, tripping the national transmission system.