Polls opened at 08:00 (05:00 GMT) in presidential and parliamentary votes.
If Erdogan wins, he will adopt major new powers that critics say will weaken democratic rule. But he faces a major challenge from centre-left candidate Muharrem Ince of the Republican People’s Party (CHP).
Turkey remains under a state of emergency imposed in the aftermath of a failed coup in July 2016.
These elections were originally scheduled for November 2019 but were brought forward by Erdogan.
Erdogan and his main rival Muharrem Ince both held huge rallies on Saturday, their final day of campaigning – and each branded the other unfit to run Turkey.
Ince, whose fiery campaigning has revitalised Turkey’s demoralised opposition, promised to push back what he characterised as a slide into authoritarian rule under Erdogan.
“If Erdogan wins, your phones will continue to be listened to… Fear will continue to reign,” he told at least a million people gathered in Istanbul. “If Ince wins, the courts will be independent.”
Ince also said that if elected, he would lift Turkey’s state of emergency within 48 hours. Emergency rule allows the government to bypass parliament, BBC reported on Sunday.
At his own rally, President Erdogan – who was prime minister for 11 years before becoming president in 2014 – used a violent metaphor to summarise his hoped-for result, asking supporters, “Are we going to give them an Ottoman slap [a technique for knocking someone out] tomorrow?”
He accused Ince – a former teacher and MP of 16 years – of lacking the skills to lead.
“It’s one thing to be a physics teacher, it’s another thing to run a country,” Erdogan said. “Being president needs experience.”
He told supporters he planned to push through more major infrastructure projects to boost the economy.
Never in its modern history has this crucial country felt so divided. And never has Recep Tayyip Erdogan faced such a tough election fight.
Turkey’s most powerful leader since its founding father Ataturk would become more powerful still if he wins, scrapping the post of prime minister and weakening parliament. But if he fails to reach 50% in the presidential vote, he’ll face a run-off, probably with Muharrem Ince, a fiery centre-left candidate who has electrified the campaign.
In the parliamentary poll, a united opposition is hoping to deprive Erdogan of his majority. Worshipped by his supporters, abhorred by his critics, this is President Erdogan’s judgement day. Nobody can tell which way it will go.
Two votes are being held on Sunday – one to choose Turkey’s next president, and another to pick members of parliament.
Around 60 million Turks are eligible to take part.
Six candidates are vying for the presidency, and if one of them wins more than 50% of the vote they will be elected outright.
If nobody hits that threshold, the top two will face off in a second-round vote on 8 July.
Erdogan will be hoping to win decisively, as a run-off vote could end in defeat or narrow his margin of victory.
In the parliamentary election, the president’s AK Party (AKP) will face a tough battle to keep its majority in the 600-seat assembly.
The contest pits a government-led coalition against an alliance of opposition parties.