Voting is under way in Turkey in a landmark referendum that will determine whether President Recep Tayyip Erdogan will be granted sweeping new powers.
Mr 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.
Two people were shot dead near a polling station in the south-east.
The cause of the incident, in Diyarbakir province, was not immediately clear but there appears to have been a dispute between two rival groups.
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.
Mr 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.
What is the case for 'Yes'?
There is just a simple "Yes" or "No" choice on the ballot
Mr 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.
"This public vote is [about] a new governing system in Turkey, a choice about change and transition," he said after casting his vote in Istanbul.
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.
Mr Erdogan voted in Istanbul accompanied by his wife and two grandchildren
The referendum, the BBC's Mark Lowen reports, is effectively one on Mr Erdogan and the Turkey he has moulded in his image: fiercely nationalist and conservative.
And what about for 'No'?
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.
They say his ability to retain ties to a political party - Mr Erdogan could resume leadership of the AK Party (AKP) he co-founded - would end any chance of impartiality.
Kemal Kilicdaroglu, leader of the main opposition Republican People's Party (CHP), told a rally in Ankara a "Yes" vote would endanger the country.
<img data-cke-saved-src="https://ichef.bbci.co.uk/news/624/cpsprodpb/16572/production/_95660519_039024231-1.jpg" src="https://ichef.bbci.co.uk/news/624/cpsprodpb/16572/production/_95660519_039024231-1.jpg" datasrc="https://ichef.bbci.co.uk/news/320/cpsprodpb/16572/production/_95660519_039024231-1.jpg" class="responsive-image__img js-image-replace" alt="The leader of Turkey's main opposition Republican People" s="" party,="" kemal="" kilicdaroglu,="" is="" greeted="" by="" supporters="" outside="" a="" polling="" station="" where="" he="" voted="" in="" the="" constitutional="" referendum="" ankara,="" 16="" april"="" width="976" height="650" data-highest-encountered-width="624" style="border: 0px; color: rgb(189, 189, 189); font-style: inherit; font-variant: inherit; font-stretch: inherit; font-family: inherit; font-weight: inherit; letter-spacing: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline; height: auto; max-width: 100%; user-select: none; width: 616.198px; transition: opacity 0.2s ease-in; display: block;">
Supporters greeted main opposition leader Kemal Kilicdaroglu when he voted in Ankara
"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.
Many Turks already fear growing authoritarianism in their country, where tens of thousands of people have been arrested, and at least 100,000 sacked or suspended from their jobs, since a coup attempt last July.
The campaign has taken place under a state of emergency imposed in the wake of the failed putsch.
Mr 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 Mr Erdogan, religious Turks have been empowered.
Relations with the EU, meanwhile, have deteriorated. Mr 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".
The draft states that the next presidential and parliamentary elections will be held on 3 November 2019.
The president would have a five-year tenure, for a maximum of two terms.