Prime Minister Ahmet Davutoglu said all possible causes were under investigation, including terrorism.
Trams and subways were idled in Istanbul -- with more than 14 million inhabitants, the largest city in the country and the fifth largest in the world. Homes and offices were left without electricity.
"I am sitting in my apartment," CNN producer Gul Tuysuz reported from Istanbul. "No lights. No electricity."
The capital, Ankara, some 250 miles from Istanbul, also was affected. The outage extended to 45 of Turkey's 81 provinces.
The semiofficial Anadolu Agency quoted Energy Minister Taner Yildiz as saying the cause of the outage was unknown. Yildiz, speaking during a visit to Slovakia, said he could neither confirm nor deny that a cyberattack had triggered the blackout, the Anadolu Agency reported.
Flightradar24, which maps real-time flight data on the Internet, tweeted that the outage was "affecting flights," and said 11 of its 16 air traffic monitoring systems receivers -- as distinct from the country's air traffic control system -- were not working.
The Turkish Electricity Transmission Co. blamed the outage on a problem with transmission lines, Anadolu Agency reported. The utility said an investigation was in progress, as were efforts to restore power, the news agency said.
The outage began at 10:36 a.m. (3:36 a.m. ET). Nearly two hours later, according to Anadolu Agency, about 15% of the power had been restored to Istanbul and Ankara, including in some subway stations. Power also was beginning to flow again to a number of provinces that had been cut off, the agency reported.
By midafternoon, Yildiz said, 90% of Istanbul's power had been restored.
"Crowded places such as metro stations have been given electricity, and we believe the rest of the country should be fully powered shortly," he said.