Turkey has been going to the polls in the long-awaited referendum that would give Recep Tayyip Erdogan sweeping new powers.
The president cast his ballot in Istanbul on Sunday morning along with his wife and two grandchildren, amid tight security.
He and his supporters argue the changes, described as the most radical in Turkey’s modern history, are needed to guarantee political stability.
Prime Minister Binali Yildirim cast his vote at a school in Izmir province, saying the result would be respected whatever the outcome.
If a “yes” vote prevails, his office is set to be abolished – with the president being given the authority to draft the budget, declare a state of emergency and issue decrees without parliamentary approval.
The main opposition leader Kemal Kilicdaroglu of the Republican People’s Party (CHP) cast his vote in Ankara. He has accused Erdogan of seeking a “one-man” regime under changes that would put the country in danger.
In turn he has been ridiculed by the president, who has equated a “No” vote with support for terrorism.
Erdogan’s opponents fear a “Yes” will lead to more repression. Last July’s failed coup sent shockwaves through Turkey, bringing bad memories of military rule.
But the president has been accused of using it to silence opponents. Media outlets have been closed, and many journalists and others sacked and often imprisoned.