Turks were voting in their first direct presidential election Sunday, a watershed event in the 91-year history of a country where the president was previously elected by Parliament.
Prime Minister Recep Tayyip Erdogan, a pious Muslim, who has dominated the country's politics for the past decade, is the strong front-runner to replace the incumbent, Abdullah Gul, for a five-year term.
Polls opened at 0500 GMT and are due to close at 1400 GMT.
Fervently supported by many as a man of the people who has ensured a period of economic prosperity, Erdogan is seen by others as an increasingly autocratic leader.
His opponents accuse Erdogan of undermining the secular legacy of Turkey's founding father Mustafa Kemal Ataturk, who based the modern state after the collapse of the Ottoman Empire on a strict separation between religion and politics.
Erdogan, who is happy to be referred to by followers as the "Sultan," has made clear he intends to be a head of state who "sweats" and exercises real power.
Erdogan is running against two other candidates. His main challenger is Ekmeleddin Ihsanoglu, a 70-year-old academic and former chief of the Organization of Islamic Cooperation who is backed by several opposition parties, including the two main ones: The republicans and the nationalists.
The third candidate is 41-year-old Kurdish politician Selahattin Demirtas, who is considered a rising star on the minority Kurdish political scene.
A candidate needs an absolute majority for victory on Sunday. If none wins enough ballots, a runoff between the top two vote-getters will be held on Aug. 24.
However, "There really is no uncertainty about this outcome. It's almost a foregone conclusion that Erdogan will win," said Sinan Ulgen of the Carnegie Center.
While many secular Turks detest Erdogan intensely, he can still count on a huge base of support from religiously conservative middle-income voters particularly in central Turkey and poorer districts of Istanbul who have prospered under his rule.
"We will write the history of the new Turkey on August 10," he told tens of thousands of cheering supporters in Ankara in one of his final election rallies.
Some 53 million people are eligible to cast votes in more than 160,000 polling stations across the country. Polls close at 1400 GMT, and only unofficial results are expected to be released on Sunday night.
After leading a bitter and divisive pre-election campaign, Erdogan sounded a more conciliatory and unifying note in his final campaign speech Saturday night.
"This country of 77 million is our country, there is no discrimination," he said. "We own this country all together."
Results are expected to come in rapidly and many suspect Erdogan is already planning a victory speech from the balcony of AKP headquarters in Ankara around midnight.
Erdogan over three months has spearheaded a lavish and immense campaign that has swamped the efforts of his rivals, holding mass rallies in almost 30 Turkish cities as his face glared on gigantic billboards at pedestrians in Istanbul at almost every street corner.
"National will, national power," reads his main election slogan.
Erdogan's main campaign advertisement - which had to be edited after a court ruled images of a woman praying violated Turkey's secular rules - has saturated national TV and shows the "people's president" leading thousands of ordinary Turks into the Cankaya.
The campaign of Ihsanoglu - a bookish former head of the Organisation of Islamic Cooperation (OIC) - has been modest by comparison and Erdogan has gleefully belittled his main rival as a dreamy academic who will get nothing done.
The third candidate has shown considerably more dynamism and his charisma, flashing grin and fondness for white shirts with rolled-up sleeves have led him to be dubbed the "Kurdish Obama" in some quarters.
Yet Demirtas' campaign will be considered a success if he musters over 10 percent of the vote.
Erdogan fighting after 2013
Erdogan endured the toughest year of his rule in 2013 and was shaken by deadly mass protests sparked by plans to build a shopping mall on Gezi Park in Istanbul that grew into a general cry of anger by secular Turks who felt ignored by the AKP.
Later in the year, stunning corruption allegations emerged against the premier and his inner circle, including his son Bilal based on bugged conversations that enthralled the country like a soap opera.
But Erdogan has come out fighting, denying the allegations and blaming a former ally turned rival, the Pennsylvania-based Turkish cleric Fethullah Gulen for launching a plot against him.
He has behaved at rallies as much like a prize fighter as a politician, aiming punches at foes like Gulen or even a critical female Turkish journalist who he denounced as a "shameless" woman.
With the election campaign coming amid Israel's offensive in the Gaza Strip, Erdogan caused huge controversy outside of Turkey by saying the Jewish State was behaving even worse than Adolf Hitler.
If he wins, Erdogan will in late August take over as president from Abdullah Gul, a co-founder of the AKP who appears to have taken a distance from the pugnacious premier and whose political future is unclear.
There will be great scrutiny on who Erdogan decides should be the new premier with Foreign Minister Ahmet Davutoglu seen as a possible choice.
AP and AFP contributed to this report