Preliminary results from Turkey's presidential election showed President Recep Tayyip Erdogan ahead with almost 60 per cent of votes, compared with about 35 per cent for opposition leader Kemal Kilicdaroglu late on Sunday.
But sources for Mr Kilicdaroglu's Republican People's Party (CHP) said the opposition leader was ahead by a narrow margin, Reuters reported.
CHP spokesman Faik Oztrak claimed his party was leading before state broadcaster TRT declared Mr Erdogan was in the lead.
Surveys had previously shown Mr Erdogan trailing Mr Kilicdaroglu, which would lead to Turkey having a new leader after more than 20 years of Erdogan rule.
Results were expected to roll in from across the country after 9pm local time (10pm UAE time).
In the Republican People's Party (CHP) stronghold of Besiktas, English student Mert Turkan, 24, from Iskenderun, told The National he voted for party leader Kemal Kilicdaroglu.
Iskenderun was hard hit in the February earthquake, which left more than 50,000 people dead — including Mr Turkan's brother, sister-in-law and nephew.
”People are against Erdogan because he failed to save our people,” he said, as he showed us video footage of him extracting his brother’s corpse from under the rubble. He said he no longer wanted to live in Turkey and hopes to emigrate to Germany.
"[My nephew] was born next to me, and he died next to me," said Mr Turkan, before displaying a tattoo on his left forearm of his nephew perched on his shoulders.
The student, who dropped out of university after the earthquake to support his family, said he supported Muharrem Ince in the last election, but is now supporting the CHP.
“I hate Recep Tayyip Erdogan,” he said. "He’s a dictator. Kilicdaroglu is trustworthy, he’s a really cool guy. Erdogan doesn’t allow anything.
A presidential candidate must secure more than 50 per cent of votes to avoid a run-off election on May 28.
Various restrictions have been put in place, including a ban on the sale of alcohol until midnight.
Media outlets are banned from reporting election-related news, commentary or predictions until after 6pm.
More than 191,000 ballot boxes have been sent out across Turkey for the about 61 million eligible voters, state media reported. Additional polling stations have been set up for tens of thousands of people displaced by February's earthquake who cannot return home to vote.
Outside a polling station in the neighbourhood of Katip Mustafa Celebi, local governor Aysel Kilic said: ”I can’t officially say who I voted for but I hope the best for Turkey.
"I hope everything will be beautiful,” she said, using a slogan of Mr Kilicdaroglu.
”The atmosphere is good, people are voting and everything is under control.”
Mehmet Sankarsan, 42, said he voted for the opposition in the hope it would improve life for Turkey’s large Kurdish minority.
“I hope Kilicdaroglu will win and he will solve the Kurdish issue and help poor people, because economically they’re in a terrible way,” he said. "I hope he will bring equality for all citizens."
Artist Bilge Keresu, 35, said she hoped an opposition win would strengthen women’s rights in Turkey.
”As women we have a lot of difficulties in Turkey," she said. "If Kilicdaroglu comes, I hope we will rejoin the Istanbul Convention and we will have our rights to live as equal citizens, the same as men as Turkey.”
Voters will also elect deputies to Turkey's 600-seat parliament.
The presidential candidates made their final pleas for votes in Istanbul and Ankara on Saturday.
Mr Kilicdaroglu, leader of the People's Republican Party (CHP), addressed a rally in the capital, where he also visited the mausoleum of Mustafa Kemal Ataturk, founder of the party and of the modern Turkish state.
Addressing supporters in the rain, Mr Kilicdaroglu said: "Ataturk was open to innovation. He embraced change with courage. Focus all your energy on building the new, not fighting the old."
Mr Erdogan, leader of the Justice and Development Party (AKP), toured three Istanbul districts on the eve of the election, including his home area of Kasimpasa, where residents affirmed their support for the president despite the economic problems now facing the country.
Speaking later at the Hagia Sofia mosque, he said: "We will emerge stronger from the ballot box."
Mr Erdogan has dismissed accusations that he would not step down if he was defeated.
“If our nation decides to make such a different decision, we will do exactly what’s required by democracy,” he said in an interview aired by more than a dozen Turkish broadcasters on Friday.
The pro-Kurdish Peoples' Democratic Party (HDP), which attracts about 10 per cent of votes and has backed Mr Kilicdaroglu, held its final rally in the Istanbul district of Yenekepi. Kurdish rap blared from loudspeakers as officials took to the stage shouting "Jin, Jiyan, Azadi" or "Woman, Life, Freedom" — a famous Kurdish slogan which has recently featured in Iran's anti-government protests.
The withdrawal of presidential candidate Muharrem Ince on Thursday is expected to boost Mr Kilicdaroglu's votes.
Turnout is traditionally strong in Turkey, with more than 81 per cent of voters taking part in the 2018 election. Millions of first-time voters will also take to the polls.
Voters in the diaspora have already broken participation records, with more than three million votes cast.