The Turkish prime minister has claimed victory in the referendum to grant President Recep Tayyip Erdogan new powers, based on unofficial results.
Binali Yildirim was speaking as the count neared completion. With about 99% of ballots counted, "Yes" was on about 51.3% and "No" on about 48.7%.
Erdogan supporters say replacing the parliamentary system with an executive presidency would modernise the country.
The two main opposition parties are challenging the results.
The Republican People's Party (CHP) demanded a recount of 60% of the votes.
A "Yes" vote could also see Mr Erdogan remain in office until 2029.
The president earlier called the head of the AK Party (AKP) he co-founded and other party leaders to congratulate them on the victory, the Anadolu news agency said.
But Deputy Prime Minister Veysi Kaynak admitted the "Yes" votes were lower than expected.
About 55 million people were eligible to vote across 167,000 polling stations, and turnout was said to have been high.
Three people were shot dead near a polling station in the south-eastern province of Diyarbakir, reportedly during a dispute over how they were voting.
How significant are the changes?
They would represent the most sweeping programme of constitutional changes since Turkey became a republic almost a century ago.
The president 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.
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.
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 - would end any chance of impartiality.
Kemal Kilicdaroglu, leader of the 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.