Iran began voting on Friday for a new president after a fiery campaign which has seen moderate ex-premier Mir Hossein Mousavi emerge as the main challenger to incumbent hardliner Mahmoud Ahmadinejad.
The interior ministry, which is in charge of organising the election, said polls opened at 8:00 am (0330 GMT) and may remain open until midnight depending on turnout among the 46-million-strong electorate. Results are expected within 24 hours after voting ends.
The country's 10th presidential election since the 1979 revolution is a close two-horse race with passions running high after three weeks of mass rallies, stormy television debates and vicious mudslinging.
While Ahmadinejad, 52, is battling for a second four-year term in office, Mousavi, 67, is seeking to make a comeback after two decades in the political wilderness.
The election campaign turned a spotlight on deep differences within the Islamic republic after four years under Ahmadinejad.
His hardline rhetoric on the nuclear standoff and against Israel has isolated the country from the West, and his expansionist economic policies have come under fire at home.
The campaign also highlighted the glaring internal divide, with towns and villages passionately backing Ahmadinejad and young men and women in big cities throwing their weight behind Mousavi.
Analysts have been reluctant to forecast a winner, suggesting the vote may mirror 2005 when the relatively unknown Ahmadinejad scored a stunning upset in a second-round runoff against heavyweight cleric Akbar Hashemi Rafsanjani.
The 2009 campaign has been marked by street demonstrations and unprecedented public animosity among the candidates who traded insults and allegations of lying and corruption on prime-time television, ratcheting up the tension.
At a final campaign rally on Wednesday, Ahmadinejad, who has frequently described the Holocaust as a myth, accused his rivals of using "tactics like Hitler" to whip up public opinion against him, the Fars news agency said.
On the streets, Iranians also used the occasion to turn political rallies into night-time parties in a country that has had little to offer in terms of nightlife during three decades of conservative clerical rule.
Standing for election alongside Ahmadinejad and Mousavi -- but also running far behind -- are reformist former parliament speaker Mehdi Karroubi and the ex-head of the elite Revolutionary Guards Corps, Mohsen Rezai.
Mousavi has pledged to work to improve relations with the outside world, although there are doubts nuclear policy would change as all strategic decisions are taken by supreme leader Ayatollah Ali Khamenei.
The election comes after new US President Barack Obama offered dialogue with Iran -- dubbed part of the "axis of evil" by his predecessor George W Bush -- following three decades of severed ties.
Mousavi has complained that Ahmadinejad's foreign policy "undermined the dignity of Iran," and along with his fellow challengers accused the president of mismanaging the economy.
Iran, OPEC's second biggest oil exporter, is currently battling rampant inflation of 24 per cent and a slump in earnings from crude oil as international prices have sunk from close to 147 dollars last year to around 72 dollars.
Ahmadinejad has left no stone unturned in seeking to bolster his image as a man of the people, accusing his rivals and their backers of dishonesty and receiving financial privileges.
The elite Revolutionary Guards -- seen to be backing Ahmadinejad -- even accused Mousavi supporters of trying to spark a "velvet revolution" by taking to the streets in their thousands dressed in green, his campaign colour.
"Such a scenario will never succeed in Iran due to the political structure of our Islamic nation. I believe that with the alertness of the people, any move for any velvet revolution will be nipped in the bud," Guards chief Yadollah Javani said in a media interview, referring to the non-violent overthrow in 1989 of the communist regime in then Czechoslovakia.
If blacksmith's son and former Tehran mayor Ahmadinejad is defeated, it will be the first time a sitting president has been ousted after a single term.
A runoff will be held on June 19 if no single candidate emerges with 50 per cent plus one vote on Friday.