Initial results were expected later in the day. The victor will assume vast new presidential powers.
Should he win the June 24 elections, Erdogan would be Turkey's first leader under the presidential system, which does away with the prime minister's post, among other changes. He attacked his opposition for lacking vision, and boasted of his achievements in office - such as new infrastructure and improved healthcare.
"There is no other party that can run Turkey", said Olcay, 33, who was on his way to vote for the incumbent in Istanbul's largely conservative Üsküdar district.
Al Jazeera's Sinem Koseoglu takes a look at the frontrunners.
Turkish President Tayyip Erdogan speaks to the press after casting his ballot at a polling station in Istanbul, Turkey, on June 24, 2018.
If the HDP exceeds the 10 percent threshold of votes needed to enter parliament, it will be harder for the AKP to get a majority.
As he cast his vote, Erdogan said the changes marked a "democratic revolution", although his opponents regard the most recent phase of his rule differently.
Selahattin Demirtas, a jailed Turkish-Kurdish politician, is also a candidate for the pro-Kurdish Peoples Democratic Party's (HDP).
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We want to improve, we want to learn. "We are ready for this match and we are united, we are working like a team and living like a family".
After the voting ends, ballots cast for the presidential candidates will be counted first. Monitors assume the role in order to confirm the votes used in the ballot boxes match up with the results transferred to the provincial stations and then to the YSK, comparing photos of the documents that are tried to be taken at every stage, again by smart phones and through sharing via social media.
The biggest threat is posed by Muharrem Ince of the secular Republican People's Party (CHP).
Under constitutional amendments approved after a controversial 2017 referendum, Turkey is making a transition from a parliamentary system to a presidential one - giving the next president expanded powers, abolishing the prime minister's post, and eliminating numerous checks and balances created to help parliament protect against the misuse of presidential powers.The changes will take effect after the elections.
President Recep Erdogan has since the attempted coup acted to reassert his position and his power, including through the imposition of emergency law that enables him to pass legislation without parliamentary scrutiny or intervention from the judiciary.
Turkey will also be electing 600 lawmakers to parliament on Sunday - 50 more than in the previous assembly.
Erdogan and AKP supporters have dismissed allegations they will tamper with the election results, highlighting that those claims contradicted the opposition's insistence that the AKP will lose its majority. "I hope that we will wake up to a more lovely day tomorrow".
"Our country needs fresh blood like Muharrem Ince, that is why I am here", said Nuray Ugurlu, 54, who said she was attending a political rally for the first time because of concern over the rapidly rising cost of living.
Votes were cast in 180,065 polling places across the country.
Nationalist candidate Meral Aksener, nicknamed the "she-wolf" by her admirers, leads the Iyi party and is seen by many as the only viable alternative to Erdogan in a country that is becoming increasingly conservative.