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 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, reports the BBC.