Turkey voted on Sunday in a historic runoff that President Recep Tayyip Erdogan entered as the firm favourite to extend two decades of his Islamic-rooted rule to 2028.

The Nato member's longest-serving leader defied critics and doubters by emerging with a comfortable lead against his secular challenger Kemal Kilicdaroglu in the first round on May 14.

The vote was nonetheless the toughest Erdogan has faced in one of the country's most transformative eras since its creation as a post-Ottoman republic 100 years ago, resulting in its first-ever presidential runoff.

Kilicdaroglu cobbled together a powerful coalition of Erdogan's disenchanted former allies with secular nationalists and religious conservatives.

Opposition supporters viewed it as a do-or-die chance to save Turkey from being turned into an autocracy by a leader whose consolidation of power rivals that of Ottoman sultans.

But Erdogan, 69, still managed to come within a fraction of a percentage point of winning outright in the first round.

His success came in the face of one of the world's worst cost-of-living crises -- and with almost every opinion poll predicting his defeat.

"I'm going to vote for Erdogan. There is no one else like him," 24-year-old Emir Bilgin said in a working-class district of Istanbul where the future president grew up playing street football.

Kilicdaroglu re-emerged a transformed man after the first round.

The former civil servant's old message of social unity and democracy gave way to desk-thumping speeches about the need to immediately expel migrants and fight terrorism.

His right-wing turn was targeted at nationalists who emerged as the big winners of the parallel parliamentary elections.

The 74-year-old had always adhered to the firm nationalist principles of Mustafa Kemal Ataturk, the military commander who formed both Turkey and Kilicdaroglu's secular CHP party.

But these had played a secondary role to his promotion of socially liberal values practised by younger voters and big-city residents.

Analysts question whether Kilicdaroglu's gamble will work.

His informal alliance with a pro-Kurdish party left him exposed to charges from Erdogan of working with "terrorists".

The government portrays the Kurdish party as the political wing of outlawed militants.

And Kilicdaroglu's courtship of Turkey's hard right was hampered by the endorsement Erdogan received from an ultra-nationalist who finished third two weeks ago.

Erdogan has been lionised by poorer and more rural swathes of Turkey's fractured society because of his promotion of religious freedoms and modernisation of once-dilapidated cities in the Anatolian heartland.

"It was important for me to keep what was gained over the past 20 years in Turkey," company director Mehmet Emin Ayaz said before voting for Erdogan in Ankara.

"Turkey isn't what it was in the old days. There is a new Turkey today," the 64-year-old said. (AFP)