The vote pitches a divided conservative camp against a moderate candidate who enjoys the backing of the reformists.[break]
Iranian supreme leader Ayatollah Ali Khamenei, who votes in the earliest moments after the opening of the polls urged people to vote en masse.
"Inshallah (God willing), the Iranian people will create a new political epic. I advise all people to vote and do so in the early hours of the morning," said the Iranian leader in a live broadcast on state television.
He also attacked US criticism of the presidential poll.
Some US officials had said they did not recognise the presidential election, he noted.
"The hell with you... " who do not agree with the way the election is being run, Khamenei said.
"The Iranian people... will do what is in their interest," he added.
US State Department spokeswoman Jen Psaki, noting the way the field had been narrowed down to six candidates, had told reporters in Washington on Thursday: "By international standards, this election is not free, fair or transparent.
"The candidates were chosen by the Guardian Council, which is unelected and... an unaccountable body. Nonetheless, the Iranian people will make some choice among the small choices that they have," she added.
Thanks to a late surge of support by the reformists, the moderate cleric Hassan Rowhani has emerged as a frontrunner with a real chance of forcing a run-off against the conservatives, analysts say.
Unofficial polls suggest that the conservative frontrunners are former foreign minister Ali Akbar Velayati, Tehran mayor Mohammad Baqer Qalibaf and chief nuclear negotiator Saeed Jalili.
Some 50.5 million people are eligible to vote on Friday for a successor to Ahmadinejad, under whose presidency Iran has been isolated internationally because of its controversial nuclear drive.
International sanctions imposed on Tehran to try to force it to give up its sensitive uranium enrichment work have sparked a deep economic crisis, which has been the focus of the election campaign.
The polls will remain open for the next 10 hours, though voting may be extended until midnight, if the interior ministry decides that it is necessary.
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