The election could grant Erdogan, 69, a new five-year term or unseat him in favour of an opposition candidate who has promised to return Turkey to a more democratic path.

If no candidate receives more than 50% of the vote, the race 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 Erdogan’s executive presidency, Associated Press reported.

If his political alliance wins, Erdogan could continue governing without much restriction.

The opposition has promised to return Turkey’s governance system to a parliamentary democracy if it wins both the presidential and parliamentary ballots.

Opinion surveys indicated that the increasingly authoritarian leader entered the election trailing a challenger for the first time.

Erdogan has ruled Turkey as prime minister or president since 2003.

Pre-election polls gave a slight lead to Kemal Kilicdaroglu, 74, the joint candidate of a six-party opposition alliance who leads the centre-left, pro-secular Republican People’s Party, or CHP.

More than 64 million people, including 3.4 million overseas voters, were eligible to vote in the elections, which come the same year the country will mark the centenary of its establishment as a republic.

Voter turnout in Turkey is traditionally strong, reflecting citizens’ continued belief in democratic balloting.

Yet Turkey has seen the suppression of freedom of expression and assembly under Mr Erdogan, and is wracked by a steep cost-of-living crisis that critics blame on the government’s mishandling of the economy.

Turkey 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.

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.

Erdogan has led a divisive election campaign, using state resources and his domineering position over media to woo voters.

He has accused the opposition of colluding with “terrorists”, of being “drunkards” and of upholding LGBTQ rights, which he claims are a threat to traditional family values.

In a bid to secure support from citizens hit hard by inflation, he has increased wages and pensions and subsidised electricity and gas bills, while showcasing Turkey’s homegrown defence and infrastructure projects.

He also extended the political alliance of his ruling Justice and Development Party, or AKP, with two nationalist parties to include a small leftist party and two marginal Islamist parties. Kilicdaroglu’s six-party Nation Alliance has pledged to dismantle an executive presidential system narrowly voted in by a 2017 referendum and return the country to a parliamentary democracy.

They have promised to establish the independence of the judiciary and the central bank, institute checks and balances and reverse the democratic backsliding and crackdowns on free speech and dissent under Erdogan.

The alliance includes the nationalist Good Party led by former interior minister Meral Aksener, a small Islamist party and two parties that splintered from the AKP, one led by a former prime minister, Ahmet Davutoglu, and the other by a former finance minister, Ali Babacan.

The country’s main Kurdish political party, currently Turkey’s second-largest opposition grouping, is supporting Kilicdaroglu in the presidential race.

Erdogan’s government in recent years has targeted the party’s leaders with arrests and lawsuits.

Voters also cast ballots to elect lawmakers for the country’s 600-member parliament.

The opposition would need at least a majority to be able to enact some of the democratic reforms it promised.

“It’s important for Turkey. It’s important for the people,” Necati Aktuna, a voter in Ankara, said.

“I’ve been voting for the last 60 years. I haven’t seen a more important election that this one.”

Large crowds gathered outside the polling stations where Mr Erdogan and Mr Kilicdaroglu cast their votes.

“We have all missed democracy so much. We all missed being together,” Mr Kilicdaroglu said, after voting at a school in Ankara, where his supporters chanted “President Kilicdaroglu”.

“From now on, you will see that spring will come to this country,” he said.

Erdogan said voting was under way “without any problems”, including in the earthquake-affected region where people were voting “with great enthusiasm and love”.

“It is my hope that after the evening’s count … there will be a better future for our country, our nation and Turkish democracy,” he said.