The poll is also taking place under a state of emergency that has seen 47,000 arrested in an unprecedented crackdown after the July 15 failed coup.
The referendum has not only captivated Turkey, but has spilled into Europe, where it has caused a political storm.
The unprecedented stability and prosperity of the AKP's first decade in power was to a large degree due the government's inclusivity - even if that inclusivity masked a deeper and more ugly power struggle.
Voters will decide Sunday whether to approve constitutional changes that would replace the parliamentary system with a presidential one, scrapping the office of the prime minister and handing over its powers to the president. Experienced leaders in the political system indicated that systemic crises threatened, not only the governability, but also the democracy of the country, as deadlocks usually provided weak spots for institutions outside the political realm to have leverage over the national will.
In fact, a Yes vote is only likely to intensify the frustrations driving that instability, and entrench Mr Erdogan's reliance on the tools of repression that increasingly characterise his rule.
"But we will do what is necessary, within the framework of the law, against those who act as agents and threaten my country from Qandil", Erdogan said, referring to a PKK base camp in northern Iraq.
He continued saying that the European Union feared the new system because Turkey would be "even stronger".
Supporters see the new system as an essential modernisation step for Turkey to streamline government. Overseas votes have already been cast in high numbers.
Trump, Xi end summit with 'tremendous' progress
Trump is free to criticize Obama's handling of Syria and the "red-line" episode. The next day, Trump ordered a military strike on a Syrian government airbase.
The vote comes at a perilous time.
Mr Erdogan gave a sense of empowerment to religious conservatives who had previously been marginalised; he reached out to the Kurds, and at the same time did not move to curtail the freedoms of the country's secular middle class.
When Germany and Netherlands barred government ministers from campaigning, Mr Erdogan accused them of behaving like Nazis.
In recent years, Erdogan's clampdown and attempts to meddle in central bank policy have alienated foreign investors, with the lira losing a fifth of its value since the failed coup alone. "We have paid dearly for these delays".
The opposition has complained of a lopsided campaign, with Erdogan using the full resources of the state and the governing party to dominate the airwaves and blanket the country with pro-"yes" campaign posters. The main opposition party has recorded more than 100 incidents of obstruction to their campaign efforts, including threats, beatings and arbitrary detentions. He could declare a state of emergency without the now required cabinet approval.
It also noted that supporters of the "no" campaign have faced "bans, police interventions, and violent scuffles at their events".
"They said that if there is a "yes" vote, it would mean there are problems here. Everyone will know their place", he said. "You can not meddle in what happens if "yes" wins or if "no" wins". Numerous dismissed public servants were trade union members, including more than 10,000 teachers who were members of the Education and Science Workers' Union.
If approved in the referendum, the changes would transfer executive powers now held by the prime minister to the president.