Turkish President Recep Tayyip Erdogan has narrowly won a referendum to expand presidential powers, which could keep him in office until 2029.
About 55 million people were eligible to vote across 167,000 polling stations, and turnout is said to have been high.
Just over 51 percent of voters have opted for a constitutional change to grant sweeping powers to the president.
The president said the "Yes" camp had secured 25 million votes in the referendum, 1.3 million ahead of "No" votes.
Erdogan supporters say replacing the parliamentary system with an executive presidency would modernise the country.
The main opposition Republican People's Party is already demanding a recount of 60 percent of the votes.
Opponents have also attacked a decision to accept unstamped ballot papers as valid unless proven otherwise.
The president called the head of the AK Party he co-founded and other party leaders to congratulate them on the victory, the Anadolu news agency said.
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 said the changes were 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 argued, would resemble those in France and the US and would 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.