Crowds of Imamoglu's supporters took to the streets of Istanbul after the unofficial results were announced. In street parties across the city, they celebrated what they saw as a vindication after Imamoglu's original victory in March was annulled and a revote ordered.

Binali Yıldırım, the ruling Justice and Development Party's candidate (AKP), had about 45%, Anadolu said.

Yıldırım appeared to concede the vote shortly after the unofficial results were announced, saying: "My rival seems to be leading the election. I congratulate him. This elections showed democracy is functioning in Turkey. I wish Ekrem İmamoglu will make good services to people. For his good services we will help him."

Erdogan congratulated Imamoglu via his official Twitter account on Sunday, but added that his party will continue to work toward its goals "without giving up democracy, rule of law, peace, wealth and stability."

The March local election marked a political earthquake for Erdogan, who has been in power since 2003. AKP lost power to CHP in the country's three largest cities: Ankara, Istanbul and Izmir.

Istanbul was a particularly painful defeat for the president, as his own political career started there.

Istanbul, the country's biggest city, has seen a huge economic boom during the early years of the Erdogan administration. As the economic crisis hit much of the Western world, foreign investors poured their money into Turkey, looking for high returns from the emerging economy.

But the flow of money started to dry up as the US economy got stronger again. Turkish economic growth flattened, inflation reached double digits and unemployment soared.

The struggling economy has turned some residents of Istanbul against AKP.