MOSCOW, June 12 (RIA Novosti) - The Iranian Election Commission has decided to keep polls open for an additional two hours on Friday due to the high turnout in presidential elections.

The polls were due to close at 18:00 local time (13:30 GMT) and lines were reported early on as Iranians chose between hard-line incumbent President Mahmoud Ahmadinejad and the leading challenger, reformist former prime minister Mir-Hossein Mousavi.

Two other candidates, Mehdi Karroubi and Mohsen Rezaei, are also on the ballot, but are not expected to receive more than a small percentage of the vote.

The contest between Ahmadinejad and Mousavi could be close, however, and it is possible voting could be extended at some polling stations. If the winner does not get more than 50% of the vote, a runoff 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.

Even with a victory for Mousavi, who was prime minister when the post was abolished by constitutional amendments In 1989, the scope for change is limited, as the president is subordinate to the supreme leader of the Islamic Republic, Ayatollah Ali Khamenei.