ISTANBUL — Voters in Turkey's landmark referendum Sunday appeared to approve constitutional changes that will greatly expand the powers of President Recep Tayyip Erdogan, according to near-complete results carried by Turkey's official news agency.
Opposition figures, however, vowed to challenge many of the ballots in the tight race.
Erdogan has long sought to broaden his powers, but a previous attempt failed after the governing party that he co-founded fell short of enough votes to pass the reforms without holding a referendum.
If officially confirmed, Sunday's outcome is expected to have a huge effect on Turkey's long-term political future and its international relations.
The results reported by the state-run Anadolu Agency — 51.3 percent of the vote to the "yes" side to 48.7 percent for the "no" side with 98 percent of the votes counted — would fall short of the sweeping victory Erdogan had sought. Still, it would cement his hold on the country's governance.
Opponents had argued the plan hands too much power to a man who they say has shown increasingly autocratic tendencies.
Erdogan supporters gathered outside the governing AK Party headquarters in Istanbul and Ankara to celebrate, with the crowd in Istanbul sending fireworks into the night sky.
Turkish Prime Minister Binali Yildirim declared victory, saying unofficial results show a win for the "yes" vote.
"We are brothers. We are one body, one nation," Yildirim said at the ruling party's headquarters in Ankara. "A new page has opened in our democratic history with this vote. Be sure that we will use this result for our people's welfare and peace in the best way."
The main opposition party, however, cast doubt on the results of the tight vote. Erdal Aksunger, vice chairman of the Republican People's Party, or CHP, said they would challenge 37 percent of the ballot boxes and accusing Anadolu's results of being inaccurate.
The opposition Peoples' Democratic Party said on its Twitter account: "Our data indicates a manipulation in the range of 3 to 4 percent."
The country's pro-Kurdish opposition party, which also opposed the constitutional changes, says it plans to object to two-thirds of the ballots.
An unprecedented decision by Turkey's Supreme Election board to accept as valid ballot papers that don't have the official stamp also drew the ire of the CHP, with the party's deputy chairman, Bulent Tezcan, saying the decision had left the referendum "with a serious legitimacy problem."
The board made the announcement after many voters complained about being given ballot papers without the official stamp. The board said the ballot papers would be considered invalid only if it was proven that they were fraudulently cast.
More than 55 million people in this country of about 80 million were registered to vote, while a further 1.3 million Turks cast their ballots abroad.
If a "yes" victory is officially announced by the country's electoral board, Turkish voters will have approved 18 constitutional changes that will replace their country's parliamentary system of government with a presidential one, abolishing the office of the prime minister and granting sweeping executive powers to the president. The changes will come into effect with the next general election, scheduled for 2019.
The changes allow the president to appoint ministers, senior government officials and half the members of Turkey's highest judicial body, as well as to issue decrees and declare states of emergency. They set a limit of two five-year terms for presidents and also allow the president to remain at the helm of a political party.
Erdogan and his supporters had argued the "Turkish-style" presidential system would bring stability and prosperity in a country rattled by a failed coup last year that left more than 200 people dead, and a series of devastating attacks by the Islamic State group and Kurdish militants.
But opponents fear the changes will lead to autocratic one-man rule, ensuring that the 63-year-old Erdogan, who has been accused of repressing rights and freedoms, could govern until 2029 with few checks and balances.
Voting in Istanbul earlier Sunday, Erdogan described the referendum as an opportunity for "change and transformation" as black-clad bodyguards with automatic weapons stood guard.
"We need to make a decision that is beyond the ordinary," Erdogan said.
The ballots themselves did not include the referendum question — it was assumed to be understood. Voters used an official stamp to select between "yes" and "no."
At one Istanbul polling station, eager voters lined up outside before it opened at 8 a.m.
"I don't want to get on a bus with no brake system. A one-man system is like that," said Istanbul resident Husnu Yahsi, 61, who said he was voting "no."
In another Istanbul neighborhood, a "yes" voter expressed full support for Erdogan.
"Yes, yes, yes! Our leader is the gift of God to us," said Mualla Sengul. "We will always support him. He's governing so well."
Erdogan first came to power in 2003 as prime minister and served in that role until becoming Turkey's first directly elected president in 2014.
The referendum 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 intimidation, including beatings, detentions and threats.
The vote comes as 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. Gulen has denied knowledge of the coup attempt.
Still, a widespread government crackdown has targeted followers of Gulen and other government opponents, branding them terrorists and a state of emergency has been imposed.
Roughly 100,000 people — including judges, teachers, academics, doctors, journalists, military officials and police — have lost their jobs in the government crackdown, and more than 40,000 have been arrested. Hundreds of media outlets and non-governmental 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 has 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.
Fraser reported from Ankara. Bram Janssen in Istanbul and Mucahit Ceylan in Diyarbakir also contributed to this report.