Iran Press/Asia: The results are a significant blow for Recep Tayyip Erdogan, who had hoped to regain control of the cities less than a year after he claimed a third term as president.
He led the campaign to win in Istanbul, where he grew up and became mayor.
But Ekrem Imamoglu, who first won the city in 2019, scored a second victory for the secular opposition CHP.
Erdogan had vowed a new era in Türkiye's megacity of almost 16 million people, but the incumbent mayor of Istanbul was on course to win more than 50% of the vote, over 10 points ahead of the president's AK Party candidate.
This was also the first time since Erdogan came to power 21 years ago that his party was defeated across the country at the ballot box.
In the capital Ankara, opposition mayor Mansur Yavas was so far ahead of his rival by 59% that he declared victory when less than half the votes were in.
Supporters blocked all the main roads in the city, waving flags and sounding their car horns.
Significantly, the CHP was also on course to win in many of Türkiye's other big cities, including Izmir and Bursa, Adana, and the resort of Antalya.
President Erdogan, 70, acknowledged the election had not gone as he had hoped, but he told supporters in Ankara it would mark "not an end for us but rather a turning point".
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