President Recep Tayyip Erdogan was hit with a stunning political defeat on Monday as his party lost control of the capital, Ankara, in local elections.
The results of the municipal balloting on Sunday show the main opposition Republican People's Party is also slightly ahead in the contest for mayor of Istanbul.
The results reflect Erdogan's weakened support as Turkey's economy has fallen into a recession.
The president's AKP Party is challenging the result in both cities.
If the electoral authorities confirm the results of the election, it would be the first change of power in 25 years for Turkey's major cities.