Some 55 million people are eligible to vote at 167,140 polling stations across the nation, which open at 7.00 am (12.00 midnight ET) in the east of the country and close at 5 pm (1400 GMT).
In a country long wracked by political turmoil - in which the military has toppled four governments, and which only a year ago suffered a failed coup - supporters claim the changes will bring lasting stability and administrative efficiency.
The outcome Sunday is unclear, with some polls showing the country divided, and the main uncertainty is whether voters are telling the pollsters their true intentions Sunday or whether they've made up their minds.
Since a failed coup attempt in July, Turkish authorities have arrested 40,000 people and suspended 120,000 from jobs in the police, military, the civil service and the private sector.
ISTANBUL (AP) - In the last day before Turkey's crucial referendum on whether to expand the president's power, both "yes" and "no" campaigners addressed flag-waving supporters Saturday in Istanbul and Ankara.
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Meanwhile, Turkey is drifting further apart from Europe, following Erdogan's recent outbursts slamming the governments in the Netherlands and Germany as "Nazis" over their restrictions on Turkish ministers' attempts to court Turkish expatriate votes.
But critics argue that Erdogan, who has been at the helm of Turkish government as prime minister or president since 2003, will simply cement his hold on power with even fewer checks and balances if the "yes" side wins.
The constitutional changes proposed in the referendum would consolidate power in his hands.
The change would increase Erdogan's influence over who runs for Parliament.
The president would appoint an unlimited number of vice-presidents. Experienced leaders in the political system indicated that systemic crises threatened, not only the governability, but also the democracy of the country, as deadlocks usually provided weak spots for institutions outside the political realm to have leverage over the national will.
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While he wants economic relations to continue, Turkey's stalled talks about joining the European Union look even more doubtful as does the deal to curb the flow of refugees from Syria travelling on to Europe. The constitutional changes would go into effect after the 2019 presidential elections. Until 2014, presidents were chosen by parliament. Under the proposed changes, Erdogan could have his term limit effectively reset and stay in power through 2029.
For Erdogan, 63, a presidential system has been a long-time dream. He also appealed to voters of other parties to approve the changes so "Turkey can leap into the future".
Supporters say a "yes" vote would streamline and modernise the country whereas opposition "no" camp fear the move would lead to increasingly authoritarian rule.
Erdogan and government officials are accused of using state resources and official functions such as openings of infrastructure projects to campaign in favor of the changes. One of the government's emergency decrees abolished sanctions against media organisations producing biased coverage in the lead up to the election, to the obvious benefit of the government, which now indirectly controls the bulk of Turkey's media.
Turkey has been hit by a series of attacks in the last 18 months blamed on ISIL and Kurdish militants.
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