Polls have closed in Turkey in a referendum that will decide whether the country changes its constitution to create a presidential system of government -- a move that would give President Recep Tayyip Erdogan sweeping new powers and could allow him to stay in office until 2029.
The future of already strained relations between the European Union and Turkey are also at stake, with analysts predicting that a victory for Erdogan could lead to an outright break in relations between Ankara and Brussels.
The 18 constitutional amendments that were voted upon in a simple "yes" or "no" vote would also weaken Turkey's parliament, eliminate the post of prime minister, and give the president more control over the judiciary.
A simple majority of votes is needed for the referendum to pass.
Less than two hours after the polls had closed, with 75 percent of the ballots counted, Turkish election officials said 54 percent of the tallied ballots were in support of expanding Erdogan's presidential powers.
Election officials also said that 86 percent of eligible voters had cast a ballot. Final results are expected to be announced later on April 16.
In Istanbul, Erdogan was applauded by supporters after he cast his ballot in a school near his home.
Erdogan told reporters that a "yes" vote was a "choice for change and transformation," and that he believes in the "democratic common sense" of Turkey's people.
Prime Minister Binali Yildirim -- who also campaigned for a "yes" vote -- cast his ballot in the western province of Izmir, saying that "whatever the result is, we will hold it in high esteem."
"We need to make a decision that is beyond the ordinary," he told reporters, adding that he hopes the country will make the "expected" decision.
Some 55 million people were eligible to vote at 167,140 polling stations across the country. Polls opened at 7 a.m. local time in the east while voting in the rest of the country began at 8 a.m. and closed at 5 p.m.
Just hours after polls opened, Turkey's state-run news agency said two people were killed in a fight between two families outside a polling station in the southeastern province of Diyarbakir.
The private Dogan news agency reported that the deadly brawl was the result of "differences in political opinion."
Turkish voters abroad cast their ballots last week.
Opinion polls published just days ahead of the vote suggested a narrow majority of Turks would vote in favor of the amendments, which would lead to the biggest change in Turkey's system of governance since the modern republic was founded in 1923.
Erdogan and his Justice and Development Party (AKP) have argued that an ongoing insurgency by Kurdish separatists, an attempted military coup last July, repeated terrorist attacks in the country, and the influx of more than 2 million refugees fleeing the war in neighboring Syria have created the need for a strong presidency that can streamline its decisions and better steer the country through its challenges.
The leadership of the opposition National Movement Party (MHP) has also called for a "yes" vote after reaching an undisclosed deal with the ruling AKP.
However, five lawmakers in the MHP have campaigned against the proposed amendments and polls have shown that as many as two-thirds of the party's support base are opposed to the measures.
Turkey's second-largest party, the Republican People's Party (CHP), has campaigned against the amendments, which would take effect in 2019 if approved.
CHP lawmaker Silina Dogan has charged that the authoritarian nature of the amendments would bring an end to Turkey's hopes of ever joining the European Union.
A pro-Kurdish opposition group, the Peoples' Democratic Party (HDP) has also campaigned against the proposed amendments, saying they are undemocratic and violate the principle of judicial independence.
Critics in Turkey have gone as far as to say the new system of government would be a kind of "elected dictatorship" without any separation of powers, leaving the parliament without legislative authority and unable to hold the president accountable for misdeeds.
Western critics also have said the amendments would concentrate too much power in the hands of the president.
Human Rights Watch has said the proposals pose a huge threat to human rights, the rule of law, and Turkey's democratic future because, if passed, they would "concentrate unchecked power" in Erdogan's hands.
The Council of Europe has said it is deeply concerned about whether the amendments would guarantee the separation of powers in Turkey, proper checks and balances between the different branches of government, or the independence of the judiciary -- adding that all were a "prerequisite for democratic societies."
European Commissioner for Human Rights Nils Muiznieks issued a report on April 12 expressing "grave concern" that the constitutional revisions would reduce the autonomy of Turkey's already weak judiciary.
Muiznieks said the proposed amendments also did not address "serious shortcomings" in Turkey's constitution on human rights and freedom of expression.
Earlier in April, German Chancellor Angela Merkel said during a visit to Ankara that the proposed amendments would amount to a "profound political transformation."
Merkel also urged that "everything should be done to ensure that separation of powers and plurality of opinion are guaranteed in Turkey."
'Burning Bridges' With EU
The campaign ahead of the April 16 vote has also been marred by controversy.
The OSCE's election monitoring group, the Office for Democratic Institutions and Human Rights, noted on April 7 that the campaign and vote itself were taking place under a declared state of emergency following the failed coup attempt of July 2016.
The OSCE monitors also noted that fundamental freedoms have been curtailed under that state of emergency, with thousands of citizens detained or dismissed from their jobs -- including civil servants, judges, journalists, and opposition party members.
Opponents of the amendments allege they have faced state suppression while supporters of the "yes" campaign have been able to use state media, facilities, and funds to organize campaign events.
Attempts by Erdogan and his allies to stage campaign rallies targeting Turkish voters who live in the EU faced restrictions or cancellations in Germany, the Netherlands, Denmark, and Switzerland -- leading to diplomatic disputes with Ankara.
On April 13, Erdogan described Europe as a "rotting continent" that was "no longer a center of democracy, human rights, and liberty but of repression, violence, and Nazism."
Marc Pierini, a former EU ambassador to Ankara who is now an analyst with Carnegie Europe, says Erdogan's Nazi jibes have outraged EU leaders to the point that he may have "burned his bridges" with Brussels "when it comes to personal relations."
Pierini tells RFE/RL that if the constitutional amendments are approved by Turkish voters, a complete break in relations between Ankara and Brussels will seem inevitable.
"We will have a system that has no equivalent in the Western world," he says. "It is more power concentrated in one man than anywhere" in the West, "a hyper-presidential system without much checks and balances. This will be really the one-man rule system and clearly in contradiction with EU norms."
Other European experts say the optimistic scenario in terms of relations between Turkey and Brussels is that the rejection of the amendments by voters, or a narrow victory for the "yes" vote, might lead Erdogan to temper his combative attitude toward the EU and try to improve relations.