Turks are today casting their votes on whether President Tayyip Erdogan should be given more powers.
Erdogan is seeking to change Turkey's parliamentary system into an executive presidency, giving him more power to appoint cabinet ministers, choose judges, sign decrees and dissolve parliament.
Around 55m Turks will vote across 167,000 polling stations on the changes in what has become a tight race.
Final polls gave a narrow lead to the "Yes" vote, and it is thought turnout will be a crucial deciding factor in the outcome.
Supporters of the constitutional changes have argued that giving Erdogan more executive authority will allow for more efficient decision-making.
Opponents are concerned that a "Yes" vote will lead to more authoritarian rule.
Under the proposed system, the role of the prime minister would be eliminated, concentrating power with the president.
Speaking at a final rally in Istanbul, Erdogan told supporters the changes would "bring stability and trust that is needed for our country to develop and grow".
He added: "Turkey can leap into the future."
Prime minister Binali Yildirim cast his vote at a school in the Izmir province, near the Aegean coast:
Başbakan Yıldırım, İzmir'in Menderes ilçesinde oyunu kullandı. pic.twitter.com/mZGREh7Vb4— T.C. Başbakanlık (@TC_Basbakan) April 16, 2017
After voting, prime minister Binali Yildirim said:
Whatever the outcome, we will give it the highest respect. Our nation's decision is the finest decision.