The Friday election will decide a successor to outgoing President Mahmoud Ahmadinejad.

Polls across Iran opened at 08:00 local time (03:30 GMT) on Friday and will close at 18:00 (13:30 GMT). If there is a massive turnout the Interior Ministry can extend voting until midnight.

Around 51 million people are eligible to cast their ballots.

Leader of Islamic Revolution Ayatollah Seyyed Ali Khamenei also casted his ballot in the earliest moments after the opening of the polls and urged people to vote en masse.

Ahead of the vote, the national opinion polls indicate that more than 60 percent of the eligible voters will take part in the key election.

Recent polls also suggest that presidential election might go for a run-off, setting the stage for a fierce race between the two top candidates.

Under the election law, a presidential candidate must obtain more than 50 percent of the votes in the initial round to win outright. Otherwise, the two top candidates will face each other in a second round of voting on June 21.

The six candidates in Iran’s presidential election are President of the Expediency Council’s Center for Strategic Research Hassan Rohani, Supreme National Security Council Secretary Saeed Jalili, Expediency Council Secretary Mohsen Rezaei, Tehran Mayor Mohammad Baqer Qalibaf, former Foreign Minister Ali Akbar Velayati, and former Telecommunications Minister Mohammad Gharazi.

Former Majlis Speaker Gholam Ali Haddad-Adel and former First Vice President Mohammad Reza Aref withdrew their candidacies earlier this week.

Concurrent with presidential poll, Iranians are casting their ballots in the fourth city and rural council elections on Friday.