Polling stations for Turkey's historic referendum opened Sunday, when voters are set to decide on the future of their country: Whether to approve or reject reforms that would concentrate power in the hands of the president.

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.

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. and are to close at 4 p.m., 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.

People were already lined up at an Istanbul polling station before it opened.

"We are here early to say 'no' for our country, for our children and grandchildren," said retired tax officer Murtaza Ali Turgut. His wife Zeynep agreed, saying: "I was going to come sleep here last night to vote at first light."

Another "no" voter, Husnu Yahsi, said: "I don't want to get on a bus with no brake system. A one-man system is like that."

In another Istanbul neighborhood, a "yes" voter expressed full support of Erdogan. "Yes, yes, yes. Our leader is the gift of God to us. We will always support him. He's governing so well," Mualla Sengul said.

In the predominantly Kurdish province of Diyarbakir, voter Mehmet Sayar didn't say which way he voted, but added: "I hope the result will be the best for our country because this referendum will determine the future of our children."

The official Anadolu news agency reported that military helicopters flew ballots and elections officers to some districts of Diyarbakir due to security reasons.

If approved in the referendum, the proposed changes will grant the president the power 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, 63, first came to power in 2003 as prime minister and served in that role until becoming Turkey's first directly elected president in 2014. He 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.

The campaign 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 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.