Erdogan had launched an all-out personal campaign to win back Istanbul, the economic powerhouse where he was once mayor, for his ruling Justice and Development Party (AKP).

With rampant inflation and economic crisis hitting households, city mayor Ekrem Imamoglu expressed confidence of a new victory and a partial result put the CHP ahead in Istanbul and the capital Ankara.

With 49 percent of ballot boxes opened, Imamoglu led with 50.05 percent of the vote against 41.2 percent for the AKP candidate Murat Kuram. In Ankara, with 29.2 percent of ballot boxes opened, the CHP mayor Mansur Yavas led with 58.2 percent against 34.1 percent for the Erdogan-backed candidate.

The CHP was also ahead in Izmir, Turkey's third city, and a party stronghold.

"Based on the data we have obtained, I can say that our citizens' faith in us has been rewarded," Imamoglu told reporters at the CHP's Istanbul headquarters.

"The picture we have seen now pleases us greatly, but no election is finalised before it is over," he added.

- Erdogan's treasure -

"I would like to thank all of us, our citizens, from the bottom of my heart for fulfilling this sacred duty."

After casting his vote with his family, Imamoglu emerged to applause and chants of "Everything will be fine", his 2019 election slogan.

Although Erdogan dominated the campaign the veteran president's personal role did not help overcome the widespread concerns over the country's economy.

"Everyone is worried about the day to day," said 43-year-old Istanbul inhabitant Guler Kaya as she voted.

"The crisis is swallowing up the middle class, we have had to change all our habits," she said. "If Erdogan wins, it will get even worse".

Vote change happens "when we cannot afford a living, when we cannot eat", Ali Faik Demir, a political scientist at Galatasaray University, told AFP.

An Imamoglu victory Sunday would make him the likely main opponent to Erdogan's ruling AKP in the next presidential election in 2028.

Erdogan has been president since 2014 and won a new term in May last year. He had called Istanbul the national "treasure" when launching his campaign to retake the city.

If Erdogan won back Istanbul and Ankara, he would have an incentive to "amend" the constitution to stand for re-election for a fourth term, warned Bayram Balci, political scientist at Sciences Po university in France.

- More than a mayor's race -

"This election will mark the beginning of a new era for our country," Erdogan said after casting his vote in Istanbul at midday on Sunday.

"Whoever wins Istanbul, wins Turkey," Erman Bakirci, a pollster from Konda Research and Consultancy, recalled Erdogan once saying.

The election was held with the country reeling from an inflation rate of 67 percent and having seen the lira currency slide from 19 to a dollar to 32 to a dollar in one year.

Armed clashes were reported in Turkey's Kurdish-majority southeast, leaving one dead and 12 wounded, a local official told AFP.

The pro-Kurdish DEM party said it had identified irregularities "in almost all the Kurdish provinces", in particular through suspicious cases of proxy voting.

Observers from France were refused access to a polling station in the region, according to the lawyers' association MLSA.

Some 61 million people were eligible to vote for mayors across Turkey's 81 provinces, as well as provincial council members and other local officials.

The opposition has been fractured ahead of the polls, in contrast with the local elections five years ago.

This time the CHP, has failed to rally support behind a single candidate.