Turkey's top election body ordered a re-run of the Istanbul mayoral election on Monday after President Recep Tayyip Erdogan's party complained about its shock defeat in the vote, the state news agency reported.
The ruling Justice and Development Party (AKP) narrowly lost Turkey's biggest city in March 31 local elections, ending the party and its predecessors' 25-year control of the metropolis.
The new election will take place on June 23, according to the state-run TRT broadcaster.
The AKP claims there were "irregularities and corruption" that required a re-run of the mayoral election, which was won by Ekrem Imamoglu, of the main opposition Republican People's Party (CHP) by just 13,000 votes.
Imamoglu said he would wait for an official declaration by the election body before reacting.
"There is no need to be in despair," he said in a message on social media.
Imamoglu received his official mandate after two weeks of recounts, sending shockwaves through the ruling party.
Erdogan, a former mayor of the city, once said winning Istanbul was like winning the whole of Turkey.
Along with the ruling party's defeat in the capital Ankara, it marked a major electoral setback for Erdogan and reflected widespread concern over the deteriorating economy.
Istanbul, with its 16 million residents, is the engine of Turkey's economy and controls a major chunk of public spending.
The CHP, which had previously called Erdogan a "bad loser", said it was holding an emergency meeting after the election body's announcement.
'Let's be calm'
Supporters of Imamoglu urged restraint on social media.
"Let's stand together, let's be calm... We will win, we will win again," a supporters group said in a statement.
Imamoglu, a soft-spoken former district mayor who struck a conciliatory tone throughout the heated race, has vowed to serve all citizens of the city, regardless of their political inclinations.
Erdogan presented the local elections as a matter of national survival, campaigning heavily even though he was not running himself.
For his supporters, Erdogan remains the strong leader that Turkey needs as it faces down internal and international security threats -- while also speaking for religiously conservative Turks who have felt historically sidelined.
His critics say he has undermined the rule of law with a sweeping crackdown on dissent and sewn division by portraying his opponents as enemies of the state.
The AKP still won the most seats nationwide, but has been damaged by Turkey's first recession in a decade, as well as record-high inflation and a currency that has lost more than 12 percent of its value this year alone.
The party did not request a re-run of the election for the Istanbul local assembly, in which it won a majority of seats.