Turkey’s state-run news agency says voter support for President Recep Tayyip Erdogan has dipped below the majority required to win re-election outright, making a May 28 run-off presidential election more likely.
With almost 91% of ballot boxes counted, Mr Erdogan had 49.9% of the vote, according to the Anadolu Agency.
His main challenger, opposition leader Kemal Kilicdaroglu, had 44.4% as the gap between the two shrank.
Votes from Sunday’s election are still getting counted across Turkey.
If no candidate secures more than 50% of the vote, the top two vote-getters will face a run-off on May 28.
Turkey’s election authority, the Supreme Electoral Board, said it was providing numbers to competing political parties “instantly” but would not make the results public until the count was completed and finalised.
Mr Erdogan has governed Turkey as either prime minister or president for two decades.
In the run-up to the election, opinion surveys had indicated the increasingly authoritarian leader narrowly trailed his challenger.
The race, which largely centred on domestic issues such as the economy, civil rights and a February earthquake that killed more than 50,000 people, had appeared to be shaping up as the toughest re-election bid of the Turkish leader’s 20-year rule.
With the partial results showing otherwise, members of Mr Kilicdaroglu’s centre-left, pro-secular Republican People’s Party, or CHP, disputed Anadolu’s numbers, contending the state-run agency was biased in Mr Erdogan’s favour.
Pre-election polling suggested he faced the toughest re-election battle of his two decades leading the Turkey, which has grappled with economic turmoil and the erosion of democratic checks-and-balances in recent years.
Polls closed in the late afternoon after nine hours of voting in the national election that could grant Mr Erdogan, 69, another five-year term or see him unseated by Mr Kilicdaroglu, who campaigned on a promise to return Turkey to a more democratic path.
If no candidate receives more than 50% of the vote, the winner will be determined in a May 28 run-off.
Voters also elected lawmakers to fill Turkey’s 600-seat parliament, which lost much of its legislative power under Mr Erdogan’s executive presidency.
The opposition has promised to return Turkey’s governance system to a parliamentary democracy if it wins both the presidential and parliamentary ballots.
More than 64 million people, including 3.4 million overseas voters, were eligible to vote in the elections, which come the same year as the country will mark the centenary of its establishment as a republic — a modern, secular state born on the ashes of the Ottoman Empire.
Voter turnout in Turkey is traditionally strong, reflecting citizens’ continued belief in democratic balloting.
The country is also reeling from the effects of a powerful earthquake that caused devastation in 11 southern provinces in February, killing more than 50,000 people in unsafe buildings.
Mr Erdogan’s government has been criticised for its delayed and stunted response to the disaster, as well as a lax implementation of building codes that exacerbated the casualties and misery.
Internationally, the elections were being watched closely as a test of a united opposition’s ability to dislodge a leader who has concentrated nearly all state powers in his hands.