Recep Tayyip Erdogan gestures at the polling station on the day of the Presidential runoff vote in Istanbul on Sunday. AFP

Turkish President Recep Tayyip Erdogan declared victory on Sunday in a historic runoff vote that posed the biggest challenge to his 20 years of transformative but divisive rule.

The 69-year-old leader overcame Turkey's biggest economic crisis in generations and the most powerful opposition alliance to ever face his party to take an unassailable lead.

Near complete results showed him leading secular opposition rival Kemal Kilicdaroglu by four percentage points.

"We will be ruling the country for the coming five years," Erdogan told his cheering supporters from atop a bus in his home district in Istanbul.

"God willing, we will be deserving of your trust."

Turkey's main cities erupted in jubilation as Erdogan spoke.

Traffic on Istanbul's iconic Taksim Square ground to a halt and huge crowds gathered outside his presidential palace in Ankara.

The opposition leader promised to make a statement later.

Turkey's longest-serving leader was tested like never before in what was widely seen as the country's most consequential election in its 100-year history as a post-Ottoman republic.

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

He pushed Erdogan into Turkey's first runoff on May 14 and narrowed the margin further in the second round.

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

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"I invite all my citizens to cast their ballot in order to get rid of this authoritarian regime and bring true freedom and democracy to this country," Kilicdaroglu said after casting his ballot on Sunday.

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

The former civil servant's message of social unity and freedoms 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 -- a revered military commander who formed 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 doubted Kilicdaroglu's gamble would work.

His informal alliance with a pro-Kurdish party that Erdogan portrays as the political wing of banned militants left him exposed to charges of working with "terrorists."

A woman casts her ballot at a polling station during the presidential elections in Istanbul on Sunday. AFP

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.

Some opposition supporters sounded defeated already, after emerging from the polls.

"Today is not like the last time. I was more excited then," Bayram Ali Yuce said in one of Istanbul's anti-Erdogan neighbourhoods.

"The outcome seems more obvious now. But I still voted."

Erdogan is 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 told reporters in Ankara.

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

His personal relationship with Russian President Vladimir Putin has also survived the Kremlin's war on Ukraine.

A supporter holds a flag with an image of Mustafa Kemal Ataturk, the founder of modern Turkey in Ankara. AP

Turkey's troubled economy is benefiting from a crucial deferment of payment on Russian energy imports that helped Erdogan spend lavishly on campaign pledges this year.

Erdogan also delayed Finland's membership of Nato and is still refusing to let Sweden join the US-led defence bloc.

Turkey's unravelling economy will pose the most immediate test for Erdogan.

Erdogan went through a series of central bankers to find one who would enact his wish to slash interest rates at all costs in 2021 - flouting conventional economics in the belief that lower rates can cure chronically high inflation.

Turkey's currency soon entered free-fall and the annual inflation rate touched 85 per cent last year.

Erdogan has promised to continue these policies and rejected predictions of economic peril from analysts.

Turkey burned through tens of billions of dollars trying to support the lira from politically sensitive falls ahead of the vote.

The supporters of the Nation Alliance's presidential candidate Kemal Kilicdaroglu hold posters in Ankara. AP

Many analysts say Turkey must now hike interest rates or abandon its attempts to support the lira.

"The day of reckoning for Turkey's economy and financial markets may now just be around the corner," analysts at Capital Economics warned.

Agence France-Presse