Over 55.3 million Turks are able to vote in the referendum on sweeping changes to the president's role which, if accepted, would grant Recep Tayyip Erdogan more power than any Turkish leader since its founder Mustafa Kemal Ataturk and his successor Ismet Inonu.
Polling stations opened in Diyarbakir and other eastern cities at 0400 GMT, while voting in Istanbul, Ankara and other cities got under way an hour later.
Opinion polls, always treated with caution in Turkey, predicted wildly divergent scenarios with analysts saying the outcome remains too close to call despite the clear advantage in resources and airtime enjoyed by the 'Yes' campaign.
As the rival sides held rallies to sway undecided voters up until the very last minute on Saturday, Erdogan confidently predicted the 'Yes' camp had victory in the bag.
But he urged people not to succumb to "lethargy" and go out and vote, saying "the stronger result the better".
"A 'Yes' that emerges from the ballot box with the highest margin will be a lesson to the West," he said in Istanbul's Sariyer district in the last of a stamina-busting sequence of rallies.
Speaking to AFP in Istanbul on Sunday, Emrah Yerlinkaya said he voted 'Yes' "to support" Erdogan.
"If we are here today, it is thanks to him. I also voted because I support the constitutional reform."
'Turning point'
If passed, the new presidential system would dispense with the office of prime minister and centralise the entire executive bureaucracy under the president, giving Erdogan the direct power to appoint ministers.
The system would come into force after the elections in November 2019.
Erdogan, who became president in 2014 after serving as premier from 2003, could then seek two more five-year mandates.
But it could also have even wider implications for the key NATO member, which for the last half-century has set its sights on joining the European Union.
Erdogan has warned Brussels that in the event of a 'Yes' vote he would sign any bill agreed by parliament to reinstate capital punishment, a move that would automatically end its EU bid.
Western reactions to the referendum outcome will be crucial, after Erdogan accused Turkey's allies of failing to show sufficient solidarity in the wake of the July 15 failed coup.
"The referendum will mark another turning point, or rather crossroads in Turkey's political history," wrote Hurriyet Daily News chief editor Murat Yetkin.
Sinan Ekim and Kemal Kirisci of the Brookings Institution think-tank said in a report the changes if agreed "would set in motion the most drastic shake-up of the country's politics and system of governance in its 94-year-long history".
'Bus with no brakes'
The opposition has cried foul that the referendum has been conducted on unfair terms, with 'Yes' posters ubiquitous on the streets and opposition voices squeezed from the media.
The poll is also taking place under a state of emergency that has seen 47,000 arrested in an unprecedented crackdown after the botched putsch.
Supporters see the new system as an essential modernisation step for Turkey but opponents fear it risks granting Erdogan authoritarian powers.
At his final rally, the standard-bearer of the 'No' camp, Republican People's Party (CHP) leader Kemal Kilicdaroglu, warned that Turkey was deciding if it wanted "to continue with the democratic parliamentary system or (move to) one-man rule".
He described the new system as "a bus with no brakes and whose destination is unknown".
A key question will be whether the ruling Justice and Development Party (AKP) can pull off a delicate balancing act in getting both nationalists and conservative Kurds to vote 'Yes' in support of the new system.
Hancer Senkom, a retired major, told AFP in Ankara he voted 'No' because he was "against this government".
"There were reforms between 2002-2004/5 saying Turkey will enter the EU, they would end terror but they haven't fulfilled anything. I have no trust in this government, absolutely."
In the Kurdish-majority southeastern province of Diyarbakir, self-employed Nihat Aslanbay voted against the reforms and hoped for a 'No' result.
"One man regime will not bring any benefits to this country. I said 'No' (because I'm) for an egalitarian constitution that also includes the Kurds, and for freedoms."
Jihadists nabbed pre-poll
After a slew of attacks over the last year blamed on Kurdish militants and jihadists, security is set to be a major issue on polling day.
Authorities in Istanbul on Friday detained five people suspected of planning an attack on polling day, following the arrest of 19 alleged Islamist extremists in the Aegean city of Izmir earlier in the week.
The Dogan news agency said a total of 49 IS suspects had been detained in Istanbul alone over the last week.
More than 33,500 police officers were slated to be on duty throughout Istanbul alone for referendum day, according to Turkish media.
Eyewitness? Submit your stories now via social or:
Email: news@pulselive.co.ke
