The Election Commission of Iran on Friday decided to allow voting for an additional two hours following heavy turnout in the presidential elections.
The polls were due to close at 6 pm (13:30 GMT), but high participation early in the day forced the Election Commission to extend the poll time, RIA Novosti news agency reported.
Long queues were seen in many places before the polling stations opened at 8 am (0330 GMT).
About 46.2 million Iranians out of a total population of 70.4 million are eligible to cast their vote and chose a new president.
Former prime minister and reformist Mir-Hossein Moussavi, Mehdi Karroubi and Mohsen Rezaei are the three candidates challenging hardline President Mahmoud Ahmadinejad.
Ahmadinejad, who cast his vote in a mosque in south-eastern Tehran, said the huge turnout showed the voters' willingness to continue the path of "pride, progress and prosperity."
If the winner does not get more than 50 per cent of the vote, a run-off between the two highest vote getters will take place next week.
Almost 500 candidates registered to run in the elections, but the Council of Guardians only allowed four onto the ballot.