Pakistan suffered a huge power cut early on Monday following a breakdown in its national grid, leaving millions of people without electricity.

Power was out in all the country's major centres, including the biggest city Karachi, the capital Islamabad, as well as Lahore and Peshawar.

Power minister Khurrum Dastagir said the grid failure followed a "frequency variation" in southern Pakistan.

He insisted this was "not a major crisis" and power would be back soon.

Pakistan often suffers from power cuts, which are blamed on mismanagement and a lack of investment in infrastructure. The last major blackout in October took hours to restore.

In a statement. the energy ministry said that at about 07:30 local time (02:30 GMT) the grid "experienced a loss of frequency, that caused a major breakdown", adding that "swift work" was taking place to revive the system.

Some grids in the country had already been restored, Mr Dastagir said. He told Reuters news agency that power would be restored by 2200 local time.

He told Geo TV that parts of the electricity were turned off overnight because the demand for energy during winter was less than in summer, when much of the country experiences very high temperatures and people use air conditioning and fans.

"In winter, the demand for electricity reduces nationwide, hence, as an economic measure, we temporarily close down our power generation systems at night," he said.

When they were turned on in the morning, "frequency variation and voltage fluctuation" were observed in southern Pakistan "somewhere between Dadu and Jamshoro" and subsequently "power generating units shut down one by one", he told the TV channel.

It meant that across the country, traffic lights went down, fans stopped and lights went off.

Rapid transit trains in Lahore - the driverless Orange Line metro - were suspended because of the power cut, transport officials told the BBC.