Voting opened on Sunday in Turkey in a landmark referendum on whether to approve reforms that would concentrate power in the hands of the president Recep Tayyip Erdogan.
The referendum will give Erdogan new powers and will decide the future of the country.
If the "yes" vote prevails, the 18 constitutional changes will convert Turkey's system of government from parliamentary to presidential, abolish the office of the prime minister and grant extensive executive powers to President Recep Tayyip Erdogan.
The changes include grant of power to the president to appoint ministers and senior government officials, appoint half the members in Turkey's highest judicial body, issue decrees and declare states of emergency.
It sets a limit of two five-year terms for presidents. The changes would come into effect with the next general elections, scheduled for 2019.
Erdogan, who called the referendum and has championed the "yes" campaign, says the proposed "Turkish style" presidential system will ensure the country no longer risks having weak governments, and insists the stability will lead to a long period of prosperity.
But opponents fear the changes will lead to autocratic one-man rule, ensuring that Erdogan, who has been accused of repressing rights and freedoms, could govern until 2029 with few checks and balances.
Polls in eastern Turkey opened at 7 a.m. (0400 GMT) and were to close at 4 p.m. (1300 GMT), while those in the more populous west were opening and closing an hour later. More than 55 million people in this country of about 80 million are registered to vote.
Some 55 million Turkish voters are eligible to cast their ballots, according to Hurriyet Daily News.
The referendum is expected to attract a high turnout, with 1.2 million young people eligible to vote for the first time.
The Supreme Board of Election has announced that 167,140 ballot boxes will be ready for voters; another 461 boxes have also been set up in prisons.
The election process for Turkish citizens living abroad ended on April 9. Over a million registered citizens voted at 120 foreign missions in 57 countries.
For those who have missed the two-week election period, 120 boxes at 31 customs gates will be open for Turkish citizens living abroad until 5 p.m. on Sunday.
The campaign for the referendum, which was launched last October, has been highly divisive and heavily one-sided, with the "yes" side dominating the airwaves and billboards across the country. Supporters of the "no" vote have complained of an atmosphere of intimidation, with the main opposition party recording more than 100 incidents of obstruction to its campaign efforts, including beatings, detentions and threats.
The vote also comes at a time when Turkey has been buffeted by problems. Erdogan survived a coup attempt last July, which he has blamed on his former ally and current nemesis Fethullah Gulen, an Islamic cleric living in the United States. A state of emergency imposed in the coup aftermath remains in effect. A widespread government crackdown has targeted followers of Gulen and other government opponents, branding them terrorists.
Roughly 100,000 people, including judges, teachers, academics, doctors, journalists and members of the military and police forces, have lost their jobs, and more than 40,000 have been arrested. Hundreds of media outlets and nongovernmental organizations have been shut down.
Turkey has also suffered renewed violence between Kurdish militants and security forces in the country's volatile southeast, as well as a string of bombings, some attributed to the Islamic State group, which is active across the border in Syria. The war in Syria led to some 3 million refugees crossing the border into Turkey. Turkey has sent troops into Syria to help opposition Syrian forces clear a border area from the threat posed by Islamic State militants.
Meanwhile, Turkey's relations with Europe have been increasingly tense, particularly after Erdogan branded Germany and the Netherlands as Nazis for not allowing Turkish ministers to campaign for the "yes" vote among expatriate Turks.
Erdogan, who first came to power in 2003 as prime minister and served in that role until becoming Turkey's first directly elected president in 2014, has long sought to expand the powers of the president. The result of Sunday's referendum will determine Turkey's long-term political future and will likely have lasting effects on its relations with the European Union and the world.