IN PHOTOS: Turkish Opposition Candidate Defeats Erdogan's Pick in Istanbul Election Rerun

The opposition candidate for mayor of Istanbul celebrated a landmark win Sunday in a repeat election that ended weeks of political tension and broke the long hold President Recep Tayyip Erdogan’s party had on leading Turkey’s largest city.

“Thank you, Istanbul,” Ekrem Imamoglu said to the tens of thousands of people who gathered to mark his victory after unofficial results showed he won a clear majority of the vote.

The governing party’s candidate, former Turkish Prime Minister Binali Yildirim, conceded moments after returns showed him trailing well behind Imamoglu, 54 percent to 45 percent.

Imamoglu narrowly won an earlier mayoral election on March 31, but Erdogan’s Justice and Development Party, AKP, challenged the vote over alleged irregularities. He spent 18 days in office before Turkey’s electoral board annulled the results after weeks of partial recounts.

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.

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