The vote will be closely watched by the European Union - which Erdogan says he still wants Turkey to join despite the accession process grinding to a halt - and the United States which has seen no improvement in ties with its North Atlantic Treaty Organisation ally under Donald Trump.
But in a situation labelled as blatant unfairness by activists, the HDP's presidential candidate Selahattin Demirtas has campaigned from a prison cell after his November 2016 arrest on charges of links to outlawed Kurdish militants. If a candidate wins just over 50 percent of the vote, he will win the presidency, but if not, there will be a runoff on July 8.
But he reckoned without Muharrem Ince, the presidential candidate of the secularist Republican People's Party (CHP), whose feisty performance at campaign rallies has galvanised Turkey's long-demoralised and divided opposition.
But halfway through Ince's rally, mainstream Turkish media switched over to a second Erdogan speech as he crisscrossed Istanbul, appearing in several districts.
But with Turkey's economic woes mounting, partly due to the lira currency's sharp decline, Mr Erdogan and his ruling Islamist-rooted AK Party are facing an unexpectedly strong challenge from a revitalised opposition. "If Ince wins, the courts will be independent", said Ince, adding he would lift Turkey's state of emergency within 48 hours of being elected.
Amid signs of a weakening economy, Erdogan in April declared that presidential and parliamentary elections would be held on June 24, 17 months earlier than planned.
"Necessary administrative and legal steps have been taken regarding claims on security issues at some polling stations in Suruc", election body chief Sadi Guven said. Religiously observant Muslims form the bedrock of Erdogan's support.
Polls for Turkey's landmark elections will open at 8am local time (05:00 GMT) on Sunday and close at 5pm (14:00 GMT).
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"If Erdogan wins, your phones will continue to be listened to".
The Iyi party places its emphasis on the restoration of the parliamentary system, based on the principles and ideals of Turkey's founder Mustafa Kemal Atatürk.
State-run Anadolu news agency said the six were detained after videos posted on social media reportedly showed them shouting expletives against Erdogan.
The constitutional overhaul would mean that Erdogan could stay in power for another two terms until 2029. If that happens it could cost Erdogan's AKP and its nationalist ally in the "People Alliance" dozens of seats - leading it to lose its parliamentary majority.
Muharrem Ince casted his vote in the city of Yalova.
The elections will be followed by the biggest change in Turkey's political system in over half a century.
Competing narratives have dominated the competitive election campaign, especially as political parties coalesced into pro- and anti-AKP coalitions, and are expected to play a large role in how people vote.
The president's critics, including the European Union which Turkey still nominally aspires to join, say Erdogan has used the crackdown to stifle dissent.