Iranians are voting to elect a new president, with all but one of the four candidates regarded as hardliners.
Opinion polls suggest Ebrahim Raisi, a conservative Shia cleric who heads the judiciary, is the clear favourite.
Mr Raisi is an ally of Iran's supreme leader and has been touted as a future possible successor.
Dissidents and some reformists have called for a boycott, saying the barring of several contenders left Mr Raisi with no serious competition.
Supreme Leader Ayatollah Ali Khamenei cast his vote early on Friday morning in Tehran and encouraged people to go the polls.
"Each vote counts ... come and vote and choose your president," he said. "This is important for the future of your country,"
There is widespread discontent among Iranians at the economic hardship they have faced since the US abandoned a nuclear deal with Iran three years ago and reinstated crippling sanctions.
The elections coincide with the latest round of talks in Vienna between Iran and world powers that are aimed at reviving the accord, which saw Iran agree to limit its nuclear programme in return for sanctions relief.
Incumbent Hassan Rouhani, a moderate who sought to engage with the West, cannot stand for re-election because he has served two consecutive four-year terms.
Who approved the candidates?
Almost 600 hopefuls, including 40 women, registered for the election.
But in the end only seven men were approved last month by the 12 jurists and theologians on the hard-line Guardian Council, an unelected body that has the ultimate decision with regard to candidates' qualifications.
Eshaq Jahangiri, Mr Rouhani's first vice-president, and Ali Larijani, a conservative former speaker of parliament, were among the prominent candidates not allowed to run.
By Thursday, three of the approved candidates - Supreme National Security Council secretary Saeed Jalili, MP Alireza Zakani, and reformist former Vice-President Mohsen Mehralizadeh - had dropped out.
Mr Jalili and Mr Zakani, who are hardliners, both endorsed Mr Raisi, while Mr Mehralizadeh said he wanted to "unify" the reformist vote - an apparent endorsement for Mr Hammati.
If no candidate wins more than 50 per cent of the vote in the first round there will be a run-off election.