A man votes at a polling station during the referendum on expanding the powers of the Turkish president Photograph: (AFP)
If the 'Yes' camp wins, President Erdogan can dispense with the office of prime minister and also appoint his own ministers
Turkey was voting Sunday to decide whether to expand President Recep Tayyip Erdogan's powers in a bitterly-contested and close referendum he said would determine the future of the country.
More than 55.3 million Turks are eligible to vote on sweeping changes to the president's role which, if accepted, would grant Erdogan more power than any Turkish leader since its founder Mustafa Kemal Ataturk and his successor Ismet Inonu.
Polling stations opened in the east of the country at 0400 GMT, while voting in Istanbul, Ankara and other cities got under way an hour later. The last polls close at 1400 GMT with first results expected shortly after.
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.
Voting in Istanbul along with his family, Erdogan predicted that "our people would walk to the future" by making the right choice.
"This referendum is a choice of change and transformation for a new administrative system in the Turkish Republic," he said.
After a stamina-busting and bitterly contested campaign that saw insults flung in both directions, Prime Minister Binali Yildirim said: "whatever choice comes out on top, our nation will make the most beautiful decision."
'Vote for destiny'
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 of July 15.
"We are voting for Turkey's destiny," said the standard-bearer of the 'No' camp, Republican People's Party (CHP) leader Kemal Kilicdaroglu.
"God willing, the result will be auspicious and we will all have the chance to determine Turkey's fundamental problems."
Devlet Bahceli, leader of the Nationalist Movement Party (MHP) which has backed the changes, said he hoped the vote would be a "major turning point in the life of our people and in Turkey."
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.
Supporters see the new system as an essential modernisation step for Turkey but opponents fear it risks granting Erdogan authoritarian powers and will hand him one man rule.
In the Kurdish-majority southeastern province of Diyarbakir, self-employed Nihat Aslanbay said he voted against the reforms.
"A 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."
But in Istanbul, voter 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."
Beyond changing the government system, the vote 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".
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.
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.
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 and 380,000 nationwide, according to Turkish media.