Turks voted on Sunday on whether to amend the constitution in a referendum seen as a tussle between a government led by conservative Muslims and secular opponents for influence over the EU candidate country's future.
A late opinion poll showed a majority of Turks backing the government's package, which includes changes that critics say will give the ruling party control over the judiciary.
Other polls have shown the result too close to call.
The vote will test support for Prime Minister Tayyip Erdogan whose AK Party has pushed political and economic reforms since coming to power in 2002, but is accused by the secular establishment of harbouring Islamist ambitions.
Turks, and investors, will eye the outcome for any clue to Erdogan's chances of forming a single-party government for a third consecutive term after an election due by July next year.
Erdogan has said the changes to a charter drafted in the 1980s after a military coup exactly 30 years ago are needed to strengthen democracy and bring Turkey closer to European norms.
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