Pakistan woke up on Monday morning to a massive power breakdown, followed by a "frequency variation" in the southern part of the country.
Power minister Khurram Dastgir Khan told reporters that power generators have been temporarily shut down in winter at night as an economic measure to save fuel costs.
"However, this morning, when the systems were turned back on, frequency variation was reported in the southern part of the country between Jamshoro and Dadu," he said, adding, "there was a fluctuation in voltage, and power-generating units were shut down one by one because of the knock-on effect. This is not a major crisis."
In a statement on Twitter, the energy ministry said that the system frequency of the national grid went down at 7:34am on Monday. "System maintenance work is progressing rapidly," the tweet added.
Imran Rana, spokesman of Karachi-based electricity business, K-Electric, said his company is "investigating the issue" of power cuts in the metropolis. According to reports, 90 percent of Karachi has been affected.
Meanwhile, many hospitals have reportedly opted to switch off their operation theatres because of the electricity shortage.
Twitter users posted videos of people walking on the railway tracks to show how trains were not functioning during the blackout.
Local media reports say it could take up to 12 hours to restore power.
In total, the power outage affected more than 220Mln people who are already in the grips of a major economic crisis.
Last year in October, the country was dark for more than 12 hours, mostly in the southern part of the country.