President Recep Tayyip Erdogan says his ruling alliance had won a "majority" in Sunday's presidential election.
Erdogan claimed a "clear lead" over his secular rival as votes were still being counted in the early hours of Monday morning.
"We don't know yet whether the election will be over in the first round, but if people take us to a second round, we will respect that too," Erdogan told his supporters in a rousing speech outside his AK Party's headquarters in Ankara.
With 99.78% of votes counted Erdogan was sitting with 49.25% according to private news agency ANKA's preliminary figures, while opposition leader Kemal Kilicdaroglu had 45.05%.
Opposition says Turkey is headed to a run-off vote
Mayor of Ankara Mansur Yavas, from Kemal Kilicdaroglu's party CHP, has said that a run-off vote on May 28 is now "highly likely" after Erdogan's share of the vote fell below 50% according to both the independent ANKA and the state-run Anadolu news agencies.
Yavas was speaking on Turkish television alongside CHP mayor of Istanbul Ekrem Imamoglu.
Imamoglu said that there were still over 7 million votes to be counted adding that the result could still turn in Kilicdaroglu's favor by Monday morning.
Neither candidate expecting swift final result
President Erdogan warned against announcing the results of votes too quickly while Kilicdaroglu predicted that it would be a long night.
"While the election was held in such a positive and democratic atmosphere and the vote counting is still going on, trying to announce results hastily means usurping the national will," Erdogan wrote on Twitter in his first message since voting ended.
Kilicdaroglu also wrote on Twitter, saying "We will not sleep tonight" and emphasizing the importance of counting every ballot.
The opposition has accused Erdogan's party of calling for recounts in areas where Kilicdaroglu had more votes. These accusations could not be verified.
Erdogan extends narrow lead as vote counting continues
With 83.31% of ballots tallied, President Erdogan saw his slim lead grow to 48.55% against Kilicdaroglu's 45.69%, according to private new agency ANKA.
At this rate, a second round of voting would be necessary to declare a winner.
According to the state-run Anadolu news agency, with 80.48% of votes counted, Erdogan's lead was at 50.43% against Kilicdaroglu's 43.77%.
Erdogan's parliamentary alliance ahead in vote
Turkish citizens also voted for their parliamentary representatives on Sunday.
With 60.36% of the votes counted, President Erdogan's Justice and Development Party (AKP) had gained 37.91% of the votes, with Kilicdaroglu's Republican People's Party (CHP) on 28.89%.
The ruling People's alliance, which includes AKP and several other smaller parties, held just over 50% of the vote share, whereas the CHP's Nation alliance was on 40%.
Erdogan slightly ahead with half of votes counted
President Recep Tayyip Erdogan held a narrow lead of 47.64% over main rival Kemal Kilicdaroglu on 46.6% based on 52.75% of ballots being counted, according to private news agency ANKA.
Conflicting tallies put Erdogan, Kilicdaroglu ahead
A partial vote count has placed presidential contender Kilicdaroglu in the lead with 47.39% of the votes against President Erdogan's 46.84% with 32.67% of the ballots counted, according to private news agency ANKA.
The state-run Anadolu Agency, however, released different figures.
They put Erdogan ahead with 52.43% against Kilicdaroglu's 41.68%, saying 45.70% of the votes had been counted.
The Kilicdaroglu's opposition party, CHP, accused Anadolu of manipulating the numbers to make it seem like Erdogan was set for victory.
"We are winning," Kilicdaroglu tweeted.
The tense race might come down to a razor-thin margin.
Polls close in tight Turkish vote
Turkish voting stations closed following in a knife-edge election that could end President Recep Tayyip Erdogan's two-decade rule and put the mostly Muslim nation on a more secular course.
Polling stations officially closed at 5 p.m. (1400 UTC) after nine hours of voting.
Reporting of results before 9 p.m. is not permitted, so the first indications of the outcome may not emerge until late evening.
However, election authorities may decide to allow local media to report on results earlier.
Opinion polls have given Erdogan's main challenger, Kemal Kilicdaroglu, who heads a six-party alliance, a slight lead.
Two polls on Friday showed him above the 50% threshold needed to win outright.
If neither wins more than 50% of the vote, a runoff will be held on May 28.
What some of the voters are saying
The campaign ahead of Sunday's pivotal elections has been characteristically divisive. As voters cast their ballots, the split was apparent in Turkey's largest city, Istanbul, of which Erdogan was mayor from 1994 to 1998.
"I see these elections as a choice between democracy and dictatorship. I chose democracy and I hope that my country chooses democracy. I have hope that Kilicdaroglu will win," 64-year-old retired health sector worker Ahmet Kalkan told the Reuters news agency.
However, long-time Erdogan supporter Mehmet Akif Kahraman said he was still backing the incumbent. "There has never been a change in my thoughts because the future is here. God willing, Turkey will be a world leader," said Kahraman.
Mehmet Ali Fakioglu, who was made homeless by the earthquake that hit Turkey in February, made a 15-hour journey back to the disaster-hit Antakya region to vote on Sunday.
Fakioglu, who has been staying with his son in Istanbul, relayed his anger about the fact that help from the government was slow to arrive.
"I will only say this, everybody should vote with their conscience at the ballot box," Fakioglu told Reuters. "We were forgotten, all of us, on that day, the second day even on the third day. Not only in Antakya, but people were forgotten in all those cities."
"People all around Turkey should keep this in mind when voting," he added.
In the capital, Ankara, Ceren was one of more than five million young voters eligible to cast ballots. She said she wanted to see change. "I was born during this government. I saw to what point we arrived. I don't want them anymore — enough," the 19-year-old told the AFP news agency.
However, many older voters remain loyal to the president, including 67-year-old Ankara resident Recep Turktan.
"What matters is not to divide Turkey," said Turktan, adding that the economic difficulties facing Turkey at present were beig felt around the world. "We will carry out our duty. I say, go on [with Erdogan]," Turktan said.