Istanbul, Turkey - Turkish citizens have begun voting in a key referendum on whether to adopt a presidential system with extended executive powers.

The constitutional changes proposed in Sunday's vote seek to transform Turkey's governance from a parliamentary system to an executive presidency, significantly expanding the powers of the top office.

People started casting their ballots in eastern Turkey at 7am (04:00 GMT), while voting in the rest of the country started at 8am (05:00 GMT). Polls will close at 4pm (13:00 GMT) in the east and 5pm (14:00 GMT) elsewhere. Unofficial results are expected later on Sunday.

READ MORE: Turkey's constitutional reform - All you need to know

A "Yes" vote in the referendum will give the president new powers to assign ministers, high-level state officials and vice-presidents, as well as half the members in the country's highest judicial body. It will also allow the president to dissolve parliament, and issue executive decrees and state of emergencies.

Here's how Turkey could change if it votes 'Yes' on April 16: https://t.co/m2RQKUTrnG pic.twitter.com/0qpD4cLWhw — Al Jazeera English (@AJEnglish) April 15, 2017

The changes are backed by President Recep Tayyip Erdogan, the ruling Justice and Development Party (AK Party) founded by him, and the leadership of the far-right Nationalist Action Party (MHP), whose parliamentary support was vital to take the amendments to a public vote.

READ MORE: Essential reading on Turkey's crucial referendum

The main opposition Republican People's Party (CHP), the pro-Kurdish People's Democracy Party (HDP) and other critics argue that the amendments will give too much power to one individual, undermining the separation of powers in the government.

After casting her vote on Sunday morning, Macide Dogan, a 67-year-old housewife, told Al Jazeera that Erdogan is a "one-in-a-million" leader and praised Turkey's development under his rule.

"I cast my 'Yes' vote as conservative Muslim against infidels who are threatening Turkey. Erdogan defies the world powers trying to challenge him," she said at a polling station on the Asian side of Istanbul.

"We remember the times [before Erdogan] when young girls with headscarves could not enter universities in this country. We remember the times when there was no proper healthcare and transportation."

Ahmet Erturk, 52, was also a "Yes" voter.

"We were stuck with coalition governments before [AK Party] came to power. God willing, we will wake up to a more stable Turkey as of tomorrow," he said.

REPORTER'S NOTEBOOK: Constitutional change and questions on power

Sezer Sarikaya, a 28-year-old logistics manager, told Al Jazeera he was voting "No".

"I will vote against the constitutional changes as I believe our parliamentary system set up by Mustafa Kemal Ataturk [the founder of modern Turkey] is good for us," he said.

Ayse Mucek, also a "No" voter, said that she feels democracy and rule of law are under threat in Turkey under the current administration.

"Today, there are widespread and arbitrary detentions in Turkey and there is no press freedom and rule of law anymore. This should stop in this referendum. I believe in it," the 65-year-old pensioner told Al Jazeera.

"I am at an old age. I don't think about my future anymore. But if a 'Yes' vote comes out of this referendum, young people should leave this country for better places."

INTERACTIVE: How will Turkey change if it votes 'Yes' on April 16?

Erdogan addressed supporters in Istanbul for the last time on Saturday, calling on citizens to vote "Yes" to the constitutional changes.

"[The new system] will strengthen the stable and secure environment our country needs to develop," he said.

In contrast, Kemal Kilicdaroglu, the leader of the CHP, urged supporters in Ankara to back the "No" campaign.

"We are going to vote for Turkey's future," he said. "Would you let your children go on a train without any brakes?"

The Turkish presidency has traditionally been a largely neutral and ceremonial role, without many political responsibilities, as specified under the current constitution.

However, Erdogan has effectively transformed the office into a more politically active one after he became Turkey's first president to be elected by popular vote in 2014 - in line with a set of prior constitutional changes passed in a similar referendum in 2010.

Turkey has been under a state of emergency since a group in the Turkish army tried to overthrow the government in a failed coup attempt in July 2016 that killed around 300 people in total.

In recent years, bomb attacks claimed by, or blamed on, Kurdish fighters' groups and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) have killed hundreds of security forces and civilians.

Turkey's relations with the European Union, particularly with Germany and the Netherlands, have been tense since last month after these two EU member states barred Turkish ministers from holding referendum campaign rallies within their borders, where millions of Turkish voters live.

Follow Umut Uras on Twitter: @Um_Uras