Services were completely suspended on one of the city’s three metro lines and heavily disrupted on a second, the state MENA news agency reported.
Energy ministry spokesman Mohamed al-Yamani told AFP that “power is gradually being restored” after a fault cut supply to “some Cairo neighbourhoods”.
But two hours later, the whole of the city centre and several outlying districts were still without electricity, as temperatures rose towards their midday high, which can often top 40 degrees Celsius (104 degrees Fahrenheit) in summer.
As consumers switch on fans and air conditioning units across the vast metropolis, supply routinely fails to meet demand in summer, forcing authorities to impose rolling power cuts neighbourhood by neighbourhood.
But Thursday morning’s outage was the most extensive in years, as investment in generation has failed to keep up with rising population numbers amid the economic woes plaguing the country since the Arab Spring of 2011.