Erdogan also congratulated Imamoglu in a tweet.

Imamoglu narrowly won Istanbul's earlier mayor's contest on March 31, but Erdogan's Justice and Development Party, AKP, challenged the election for alleged voting irregularities. He spent 18 days in office before Turkey's electoral board annulled the results after weeks of partial recounts.

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The voided vote raised concerns domestically and abroad about the state of Turkish democracy and whether Erdogan's party would accept any electoral loss. AKP has governed Turkey since 2002.

"You have protected the reputation of democracy in Turkey with the whole world watching," Imamoglu, his voice hoarse after weeks of campaigning, told supporters.

Losing Istanbul is much more than ceding control of Turkey's largest city and commercial powerhouse. The mayor's job was the springboard for Erdogan's own political career, and if Imamoglu performs well in the position, then the President may find himself with a future challenger.

"With the elections out of the way, the key question is how the Erdogan administration reacts to Imamoglu's victory," said Piotr Matys, a London-based analyst at Rabobank. "It would be rational to focus on economic reforms, but instead the administration could concentrate on preventing Imamoglu from building a much stronger position ahead of parliamentary and presidential elections in four years."

There had been speculation that Erdogan or the opposition might push for early elections in the case of an Imamoglu victory, but the president's nationalist ally, Devlet Bahceli of the MHP party, dismissed the prospect.

Following his now second victory, tens of thousands of people erupted in mass celebration across Istanbul, including outside the offices of the Republican People's Party, which backed Imamoglu.

Jubilant supporters chanted "Mayor again! Mayor again!" Others hung out of cars, blaring horns and waving red-and-white Turkish flags.

Erdogan's ruling party still controls 25 of Istanbul's 39 districts and a majority in the municipal assembly. Imamoglu will have to work with those officeholders to govern the city and promised to work with his political opponents.

AKP also lost control of the capital city of Ankara in Turkey's March local elections, which were held as the country faced an economic downturn, battled high inflation and two credit rating downgrades in the past year.

Melahat Ugen said she switched her vote to the opposition because she could not afford to cover basic expenses.

"I've certainly never voted left before," she said. "But I'm 62, and a bag of onions costs too much. Everything is imported and we can't afford it."

Analysts say the result would increase pressure on Erdogan's government, which is grappling with a shaky economy and multiple international crises.

"The significance of Ekrem Imamoglu's win in Istanbul cannot be understated.... he represents a much-needed change in political discourse," Lisel Hintz, an assistant professor of International Relations at Johns Hopkins University SAIS, said.

Hintz said the mayor-elect withstood a divisive campaign by the government and prevailed with a positive message.

"We now have to wait and see whether Imamoglu's tenure as mayor will be interfered with in any way, whether by cutting off funding and hampering his office's ability to provide services or by removing him under some legal pretext," Hintz said.

Addressing Erdogan in his speech, Imamoglu said, "I'm ready to work with you" to solve Istanbul's problems. The President has previously signaled an unwillingness to do so.

Istanbul, a city of more than 15 million, draws millions of tourists each year and is Turkey's commercial and cultural hub. Straddling Europe and Asia, Istanbul accounted for 31 per cent of Turkey's GDP in 2017.

Bloomberg, AP

AP