Turks began voting on Sunday with Tayyip Erdogan poised to become the country's first elected president, fulfilling his dream of what he calls a "new Turkey" and his opponents say will be an increasingly authoritarian nation.
A victory for Erdogan would seal his place in history after more than a decade as prime minister in which Turkey has emerged as a regional economic power, riding a wave of religiously conservative support to transform the secular republic founded by Mustafa Kemal Ataturk in 1923.
But his critics warn that a President Erdogan, with his roots in political Islam and intolerance of dissent, would lead the NATO member and European Union candidate further away from Ataturk's secular ideals.
In one central Istanbul district where opposition to Erdogan is strong, some voters said they did not want an authoritarian president, but one who would be non-partisan and inclusive.
"We don't want a president who is authoritarian and tainted, we want one who defends the parliamentary system and the people's interests," said architect Ahmet Kensoy, 62. "He should be independent and impartial, embracing all of society."
Some 53 million Turks were eligible to vote from 8am but compared to the previous local election in March there were initially few people at polling stations in central Istanbul. Voting ends at 5pm.
Opinion polls put Erdogan, 60, far ahead of two rivals competing for a five-year term as president. Parliament has in the past chosen the head of state but this was changed under a law pushed through by Erdogan's government.
He has set his sights on serving two presidential terms, keeping him in power past 2023, the 100th anniversary of the secular republic. For a leader who refers frequently to Ottoman history in his speeches, the date has special significance.
"God willing a new Turkey will be established ... a strong Turkey is rising again from the ashes," Erdogan said on Saturday in his final campaign speech in the conservative stronghold of Konya in central Turkey.
"Let's leave the old Turkey behind. The politics of polarisation, divisiveness and fear has passed its expiry date," he told an enthusiastic crowd of thousands who waved Turkish and Erdogan campaign flags and chanted his name.
The prime minister has promised to exercise the full powers granted to him by current laws, unlike his predecessors who have played a mainly ceremonial role. But he also plans to change the constitution to establish a fully executive presidency.
The current constitution, written under military rule after a 1980 coup, would enable him to chair cabinet meetings and appoint the premier and members of top judicial bodies including the constitutional court and supreme council of judges.
Opinion polls put Erdogan's support at 55-56%, giving him the majority he needed to win on Sunday. If there is no outright winner, a second round will be held on Aug. 24.
Surveys placed him some 20 points ahead of the main opposition candidate, Ekmeleddin Ihsanoglu. Selahattin Demirtas, head of the pro-Kurdish left-wing People's Democratic Party, is seen winning just under 10% of the vote.
Turks living abroad have been able to vote at the country's airports for the last two weeks. One man voting at Istanbul's Ataturk Airport on Saturday cited the strong economic growth and development which Turkey has experienced since his party came to power in 2002 as a strong reason to vote for Erdogan.
"Every year I come to Turkey for holidays and I can observe the immense change it has undergone. Roads, bridges, the services carried out in state institutions," said Halis, 46, who lives in Cologne, Germany with his wife, who wears a headscarf signalling Muslim piety. "How could they call someone who has done so much good for his country a dictator?"
Erdogan's ruling AK Party scored a clear victory in local elections in March and a triumph on Sunday would emphatically put an end to the toughest year of his time in power.
He was shaken by nationwide anti-government protests last summer, and months later, Erdogan and his inner circle were targeted by a corruption investigation and a power struggle with his former ally, U.S.-based cleric Fethullah Gulen.
He accuses Gulen of seeking to overthrow him and has pledged as president to continue purging institutions such as the police and judiciary where Gulen is believed to wield influence.
A man casting his ballot at Ataturk Airport on Saturday voiced his concerns about what he said was "one-man rule getting stronger every day".
"I don't see a place for myself in that new Turkey Erdogan keeps talking about, that's why I stay living abroad," said Umit, 33, who lives in Dubai. "A lot of the things that happen here do not happen in democratic countries. I live in an Islamic country, yet I feel more free."