Voting is under way in Turkey in a landmark referendum that will determine whether President Recep Tayyip Erdogan will be granted sweeping new powers.
Mr Erdogan is seeking to replace Turkey’s parliamentary system with an executive presidency.
Supporters say the move would streamline and modernize the country, but opponents fear it could lead to greater authoritarianism.
A “yes” vote could also see Mr. Erdogan remain in office until 2029.
On Saturday, Turkish politicians made their final appeals to voters preparing to cast their ballots on one of the most sweeping programmes of constitutional change since Turkey became a republic almost a century ago.
Some 55 million people are eligible to vote across 167,000 polling stations, with the results expected to be announced late on Sunday evening.
If the referendum vote falls in Mr Erdogan’s favour, it would give him vastly enhanced powers to appoint cabinet ministers, issue decrees, choose senior judges and dissolve parliament.
Mr Erdogan said the changes were needed to address the security challenges faced by Turkey, and to avoid the fragile coalition governments of the past.
Speaking at one of his final rallies in Istanbul’s Tuzla district, Mr Erdogan told supporters that the new constitution would “bring stability and trust that is needed for our country to develop and grow”.
“Turkey can leap into the future,” he said.
Mr Erdogan assumed the presidency, a largely ceremonial position, in 2014 after more than a decade as prime minister.
The referendum on constitutional change would abolish the post of Prime Minister altogether allowing the president to bring all state bureaucracy under his control.
The president says the new system will resemble those in France and the US and will bring calm in a time of turmoil marked by a Kurdish insurgency, Islamist militancy and conflict in neighbouring Syria that has led to a huge refugee influx.
The campaign, which has polarized the country, takes place under a state of emergency which was imposed following a failed coup last July. A government crackdown since then has seen tens of thousands of people arrested.
Opponents and critics of the proposed changes fear the move would make the president’s position too powerful, arguing that it would amount to one-man rule, without the checks and balances of other presidential systems.
They say his ability to retain ties to a political party – Mr Erdogan could resume leadership of the AK Party (AKP) he co-founded – would end any chance of impartiality.
Kemal Kilicdaroglu, leader of the main opposition Republican People’s Party (CHP), told a rally in Ankara that a “yes” vote would endanger the country.
“We will put 80 million people on to a bus… we don’t know where it is headed. We are putting 80 million on a bus with no brakes,” he said.
The referendum has a simple “yes” or “no” choice on whether to endorse parliament’s approval of a new draft constitution.
The draft states that the next presidential and parliamentary elections will be held on 3 November 2019.
The president would have a five-year tenure, for a maximum of two terms.