KARACHI — A breakdown in Pakistan's national power grid plunged the country of 212 million people into darkness on Saturday night, officials said.

"A countrywide blackout has been caused by a sudden plunge in the frequency in the power transmission system," Pakistan's Power Minister Omar Ayub Khan said on Twitter. He asked people across the country to remain calm.

A major technical fault in Pakistan's power generation and distribution system caused the 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.

This is Pakistan's most widespread power shutdown in the country since 2015.

In a statement, the Ministry of Energy said that, according to an initial report, there had been a fault at the Guddu Thermal Power Plant in Pakistan's southern Sindh province, which had caused power plants across the country to shut down.

In Karachi, witnesses reported seeing long queues at gas stations as people rushed to buy petrol for their home generators, which had been running overnight.

"There are long lines outside petrol pumps in the city, cars are queuing as people buy fuel for their back up generators. I was in the line, people have been waiting for hours with petrol cans in hand," said Akbar Saifi, a resident in Karachi.

Efforts are now under way to restore power to various parts of the country. Large swaths of Karachi, the largest city in Pakistan, still do not have power, according to information shared by K-Electric, the company supplying power to the city.

At 6:44 a.m. local time on Sunday, the minister for Energy Omar Ayub Khan tweeted that power had been restored to major parts of the capital city of Islamabad.

Abdullah Khan, spokesperson of PIA, the main airline of Pakistan said that all flight operations remain functional despite the power outage. "All major airports in the country have back up generators," he said.

Power breakdowns are not uncommon in Pakistan and most major hospitals, airports and other institutions have their own generators. Those who can afford to often keep petrol-run generators at homes in case of power cuts.

The blackout was initially reported on social media by residents of major urban centers, 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.

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.

"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. — Agencies