Polling stations across Turkey opened for a hotly-contested referendum which could place sweeping new powers into the hands of President Tayyip Erdogan, radically changing the country's political system.
Opinion polls have shown a narrow lead for a "yes" vote, which would replace parliamentary democracy with an all-powerful presidency, and could see Erdogan in office until at least 2029.
The outcome of the referendum will also shape Turkey's relationship with the EU - while Turkey has curbed the flow of migrants into the bloc, Erdogan says he may review the deal after the vote.
Around 55 million people are eligible to vote at 167,140 polling stations across the country, with voting starting at 7am or 8am.
Turkish voters abroad have already voted.
The referendum has bitterly divided the nation, with Erdogan and his supporters saying the changes are needed in order to confront new security and political challenges the country faces.
His opponents say it is another step towards authoritarianism in Turkey, after around 40,000 people have been arrested, and 120,000 sacked or suspended from their jobs in a crackdown following a failed coup last year.
Turkey's Prime Minister Binali Yildirim said the decision is up to the people as he cast his vote on Sunday morning.
"Whatever the result is, we will hold it in high esteem," he told reporters. "The decision of our nation is the most beautiful decision."
The crowd in the polling station chanted, "Turkey is proud of you."
Both Yildirim and President Recep Tayyip Erdogan have led the campaign for a "yes" vote in the months leading up to the referendum.