More than 64.1 million people are registered to vote in Turkey, including about 1.76 million who already cast their ballots abroad and 4.9 million first-time voters.
A total of 191,885 ballot boxes have been set across the country.
Every voter will be casting two ballots, one for the president and the other for parliamentarians, both of whom will serve five-year terms.
More than 30 political parties and 150 independent parliamentary candidates are competing in the elections.
There are five multiparty blocs in the running: the People’s Alliance, Nation Alliance, Ancestral Alliance, Labor and Freedom Alliance, and Union of Socialist Forces Alliance.
The polls opened at 8 am (05:00 GMT) and will close at 5 pm (14:00 GMT).
Media organizations are barred from reporting partial results until an embargo is lifted at 9 pm (18:00 GMT), Al Jazeera reported.
If no candidate secures more than half the votes in the first round of voting, a May 28 run-off will be held.
The three presidential candidates are Sinan Ogan, candidate of the nationalist Ancestral Alliance (ATA), Erdogan, and Kemal Kilicdaroglu who heads the left-wing, secular CHP.
Turkey transitioned from a parliamentary to a presidential system in July 2018, a month after Erdogan won the presidential election.
The powers of the president were broadened after a 2017 referendum approving the change to an executive presidency.
The post of prime minister was abolished and the president also became the head of government.
In the new system, voters elect the president directly.
Presidential candidates are nominated by parties that received at least 5 percent of votes in the previous parliamentary polls or have 20 seats in parliament.