Voting is under way in Turkey in a landmark referendum that will determine whether President Recep Tayyip Erdogan will be granted sweeping new powers.
Erdogan’s supporters say replacing the parliamentary system with an executive presidency would modernise the country but opponents fear it could lead to greater authoritarianism.
A “Yes” vote could also see Mr Erdogan remain in office until 2029.
There have been reports of gunfire at a polling station in the south-east.
Two people were killed and one wounded in the incident in Diyarbakir province, the Hurriyet daily reports, saying the violence was sparked by “differing political views”.
About 55 million people are eligible to vote across 167,000 polling stations, with the results expected to be announced late on Sunday evening.
Opinion polls suggest a narrow lead for “Yes”.
They would represent the most sweeping programme of constitutional changes since Turkey became a republic almost a century ago.
Erdogan would be given vastly enhanced powers to appoint cabinet ministers, issue decrees, choose senior judges and dissolve parliament.
The new system would scrap the role of prime minister and concentrate power in the hands of the president, placing all state bureaucracy under his control.
Erdogan says the changes are needed to address Turkey’s security challenges nine months after an attempted coup, and to avoid the fragile coalition governments of the past.
The new system, he argues, will resemble those in France and the US and will bring calm in a time of turmoil marked by a Kurdish insurgency,
Islamist militancy and conflict in neighbouring Syria, which has led to a huge refugee influx.
Speaking at one of his final rallies in Istanbul’s Tuzla district, Erdogan told supporters the new constitution would “bring stability and trust that is needed for our country to develop and grow”.
Critics of the proposed changes fear the move would make the president’s position too powerful, arguing that it would amount to one-man rule, without the checks and balances of other presidential systems.
Kemal Kilicdaroglu, leader of the main opposition Republican People’s Party (CHP), told a rally in Ankara a “Yes” vote would endanger the country.
“We will put 80 million people… on a bus with no brakes,” he said.
“No” supporters have complained of intimidation during the referendum campaign and that Turkey’s highly regulated media has given them little coverage.
A divisive campaign has ended and Turkey now faces the biggest political choice in its modern history.
One voter told a BBC reporter that a “Yes” would lead Turkey into dictatorship – and that for his grandchildren’s future he had voted “No”.
Another said he had backed “Yes” for a stronger republic and that “the outside world is against Turkey”.
Turkey’s polarisation runs deep. And whichever way this goes, one half of the country will feel defeated.
The campaign has taken place under a state of emergency imposed in the wake of the failed putsch.
Erdogan assumed the presidency, meant to be a largely ceremonial position, in 2014 after more than a decade as prime minister.
This once stable corner of the region has in recent years been convulsed by terror attacks and millions of refugees, mostly from Syria, have arrived.
At the same time, the middle class has ballooned and infrastructure has been modernised. Under Erdogan, religious Turks have been empowered.
Relations with the EU, meanwhile, have deteriorated. President Erdogan sparred bitterly with European governments who banned rallies by his ministers in their countries during the referendum campaign. He called the bans “Nazi acts”.
In one of his final rallies, he said a strong “Yes” vote would “be a lesson to the West”.