AK Party supporters wave party flags and a flag with pictures of their mayoral candidate Binali Yildirim at an election kiosk in Istanbul, Turkey, June 22, 2019. REUTERS/Murad Sezer

ISTANBUL: Millions of Istanbul residents began voting on Sunday in a re-run of a mayoral election that has become a referendum on President Tayyip Erdogan's policies and a test of Turkey's ailing democracy.