Iran chooses a new president on Friday in an election the reformists hope their sole candidate will win in the face of divided conservative ranks, four years after the disputed re-election of Mahmoud Ahmadinejad.
More than 50.5 million people are eligible to vote for the man - no women candidates were approved - to succeed Ahmadinejad, who is barred from standing for a third consecutive term under the constitution.
Polling stations open at 8:00 am (1130 HKT) and close 10 hours later, although if there is a massive turnout the interior ministry can extend voting until midnight.
At the same time as choosing a new president from six candidates, voters will also pick municipal councillors.
Iran’s supreme leader Ayatollah Ali Khamenei, who has called for a large turnout but not publicly stated his preference for any single candidate, will vote early.