Turks were voting on Sunday in a presidential runoff that could see Recep Tayyip Erdogan extend his rule into a third decade.
Erdogan, 69, defied opinion polls and came out comfortably ahead with an almost five-point lead over his rival Kemal Kilicdaroglu in the first round on May 14.
But he fell just short of the 50% needed to avoid a runoff, in a race with profound consequences for Turkey itself and global geopolitics.
His unexpectedly strong showing amid a deep cost of living crisis, and a win in parliamentary elections for a coalition of his conservative Islamist-rooted AK Party (AKP), the nationalist MHP and others, buoyed the veteran campaigner who says a vote for him is a vote for stability.
The election will decide not only who leads Turkey, a NATO-member country of 85 million, but also how it is governed, where its economy is headed after its currency plunged to one-tenth of its value against the dollar in a decade, and the shape of its foreign policy, which has seen Turkey irk the West by cultivating ties with Russia and Gulf states.
Voting started at 8 a.m.
(0500 GMT) and will finish at 5 p.m.
The outcome was expected to start becoming clear by early evening.