Iranians are voting in a presidential election where the hardline protégé of Supreme Leader Ayatollah Ali Khamenei is expected to triumph.
State- opinion polls put judiciary chief Ebrahim Raisi as the frontrunner in a field of just four candidates.
Former Central Bank chief Abdolnasser Hemmati is running as the race’s moderate candidate but has not inspired the same support as outgoing President Hassan Rouhani.
If elected, Mr Raisi would be the first serving Iranian president sanctioned by the US government even before entering office over his involvement in the mass execution of political prisoners in 1988.
He is also known for his time as the head of Iran’s internationally criticised judiciary - one of the world’s top executioners.
It also would firmly put hardliners in control across the Iranian government as negotiations in Vienna continue over trying to save Tehran’s tattered nuclear deal with world powers, as it enriches uranium to the closest point yet to weapons-grade levels.
Tensions remain high with both the US and Israel, which is believed to have carried out a series of attacks targeting Iranian nuclear sites and assassinating the scientist who created its military atomic programme decades earlier.
Polls opened at 7am local time for the vote, which has seen widespread public apathy after a panel under Mr Khamenei barred hundreds of candidates, including reformists and those aligned with Mr Rouhani.
Mr Khamenei cast the ceremonial vote from Tehran, where he urged the public to take part.
“Through the participation of the people the country and the Islamic ruling system will win great points in the international arena, but the ones who benefit first are the people themselves,” Mr Khamenei said. “Go ahead, choose and vote.”
Mr Raisi, wearing a black turban that identifies him in Shiite tradition as a direct descendant of Islam’s Prophet Muhammad, later voted from a mosque in southern Tehran, waving to those gathered to cast ballots.
The cleric acknowledged in comments afterwards that some may be “so upset that they don’t want to vote”.
“I beg everyone, the lovely youths, and all Iranian men and women speaking any accent or language from any region and with any political views, to go and vote and cast their ballots,” Mr Raisi said.
There are more than 59 million eligible voters in Iran, a nation home to over 80 million people.
However, the state-linked Iranian Student Polling Agency has estimated a turnout of just 42 per cent, which would be the lowest since the country’s 1979 Islamic Revolution.
Additional reporting by the Associated Press.