Turkish President Tayyip Erdogan, Istanbul's ousted mayor Ekrem Imamoglu and the country's former Prime Minister Binali Yildirim have cast their votes on Sunday (June 23) in a re-run of a mayoral election that has become a referendum on Erdogan's policies and a test of Turkey's ailing democracy.
Who wins is seen as significant for Erdogan's future.
In the initial March 31 vote, the opposition Republican People's Party (CHP) candidate secured a narrow victory over Erdogan's AK Party (AKP) in Turkey's largest city, a rare electoral defeat for the president.
Imamoglu had been in office for less than three weeks when, after a series of appeals by Erdogan and the AKP - the High Election Board (YSK) annulled the vote and called a re-run.
Polling stations across Istanbul opened at 8:00 a.m.
(0500 GMT), with 10.56 million people registered to vote in a city which makes up nearly a fifth of Turkey's 82 million population.
Voting ends at 5:00 p.m.
Results will be announced in the evening.
Erdogan has repeated his line that "whoever wins Istanbul wins Turkey." A second loss in the city, where in the 1990s he served as mayor, would be embarrassing for Erdogan and could weaken what until recently seemed to be his iron grip on power.