Turks have backed President Recep Tayyip Erdogan's call for sweeping new presidential powers, partial official results of a referendum indicate.
With about 98% 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.
Opponents have attacked a decision to accept unstamped ballot papers as valid unless proven otherwise.
A "Yes" vote could also see Mr Erdogan remain in office until 2029.
The president has now 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.
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.
About 55 million people were eligible to vote across 167,000 polling stations, and turnout is said to have been high.
Deputy Prime Minister Veysi Kaynak said the "Yes" votes were lower than expected. "But still they are ahead," he added.