Erdogan defied opinion polls and came out ahead with an almost five-point lead over Kemal Kilicdaroglu in the first round on May 14. But he fell just short of the 50% needed to avoid a runoff.
Voting started at 8:00 am (0500 GMT) and will finish at 5:00 pm (1400 GMT). The outcome was expected to start becoming clear by early evening, Reuters reported.