The polls have opened in the election that will decide a successor to Iranian President Mahmoud Ahmadinejad.
More than 50 million eligible voters are choosing from the six candidates who remain in the race – a moderate, four conservatives and a hardliner.
All are loyal to Supreme Leader Ayatollah Ali Khamenei, who has final say on all state matters.
Cleric Hasan Rowhani has moved to the forefront as the moderates’ choice after another pro-reform candidate pulled out on Tuesday.
Mr Ahmadinejad cannot run for a third consecutive term in office.
As the polls opened, arguments over whether to boycott the ballot still raged at coffee shops, kitchen tables and on social media among many liberal-leaning Iranians.
The choice – once easy for many who turned their back in anger after years of crackdowns – has been suddenly complicated by an unexpected chance to perhaps wage a bit of payback against Iran’s rulers.
The rising fortunes of former nuclear negotiator Mr Rowhani have brought something of a dilemma for many Iranians who faced down security forces four years ago.
Should they stay away from the polls in a silent protest or jump back into the mix in a system they claim has been disgraced by vote rigging?
Which way the scales tip could set the direction of the election and the fate for Mr Rowhani, a cleric who is many degrees of mildness removed from being an opposition leader.
But he is still the only fallback option for moderates in an election that once seemed preordained for a pro-establishment loyalist.
Iran’s presidency is a big prize, but not a crown jewel. The president does not set major policies or have the powers to make important social or political openings.
That rests with the ruling theocracy and its protectors, led by the immensely powerful Revolutionary Guard
But for liberal-leaning Iranians, upsetting the leadership’s apparent plans by electing Mr Rowhani could open more room for reformist voices.
It would mark a rare bit of table-turning after years of punishing reprisals for the 2009 protests, the worst domestic unrest in Iran since the 1979 Islamic Revolution.
“Rowhani raises a lot of interesting questions,” said Scott Lucas, an Iranian affairs expert at Birmingham University.
“Among them, of course, is whether he gets Iranians who have rejected the system to then validate the system by voting again.”
And there are many other factors at play.
Many Iranians say they are putting ideology aside and want someone who can stabilise the sanctions-battered economy – one of the roles that does fall within the presidential portfolio.
This could boost candidates such as Tehran Mayor Mohammad Bagher Qalibaf, who is seen as a financial steady hand.
The rest of the candidates approved to run by election overseers – from more than 680 hopefuls – are stacked heavily with pro-establishment figures such as hardliner Saeed Jalili, the current nuclear negotiator.
Among those blocked from the ballot was former President Akbar Hashemi Rafsanjani, who is one of the patriarchs of the Islamic Revolution.
The vetting appeared aimed at bringing in a pliant and predictable president after disruptive internal feuds with Mr Ahmadinejad, who upended Iran’s political order by trying to challenge the authority of Ayatollah Khamenei.
The desire for calm is also fuelled by the critical months ahead, which could see the resumption of nuclear talks with the US and other world powers.
At the final rallies, Mr Rowhani’s supporters waved his campaign’s signature colour purple – a clear nod to the now-crushed Green Movement and its leader, Mir Hossein Mousavi, who has been under house arrest for more than two years.
Iranians traditionally have shown high interest in voting. The average reported turnout in the past 10 presidential election is more than 67%, with officials saying there was 85% participation in 2009.