The Turkish prime minister has claimed victory in the Referendum to grant President Recep Tayyip Erdogan new powers, based on unofficial results.
Erdogan supporters say replacing the parliamentary system with an executive presidency would modernise the country.
The two main opposition parties are challenging the results.
A "Yes" vote could also see Mr Erdogan remain in office until 2029.
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.