If neither of them wins more than 50% of the vote on Sunday, a runoff will be held on May 28.
Polling stations in the election, which is also for a new parliament, will close at 5 p.m. (1400 GMT).
"I chose democracy and I hope that my country chooses democracy," said Kalkan, a retired health sector worker.
Erdogan, 69 and a veteran of a dozen election victories, says he respects democracy and denies being a dictator.
The election takes place three months after earthquakes in southeast Turkey killed more than 50,000 people.
[1/14] A voter casts a ballot at a polling station in Ankara, Turkey May 14, 2023.
Human Rights Watch, in its World Report 2022, said Erdogan's government has set back Turkey's human rights record by decades. »