Iran's state-funded broadcaster Press TV reported voting hours were extended into Friday evening because of high turnout for presidential elections.
Voters are choosing from former Tehran Mayor Mohammed Qalibaf, former Foreign Minister Ali Akhbar Velayati, chief nuclear negotiator Saeed Jalili, former nuclear negotiator Hassan Rohani, former Oil Minister Mohammad Qarazai and former military commander Mohsen Rezaei
Rohani is the only reformist candidate left in the race. The rest are considered close allies of Khamenei.
Presidential candidates were cleared by the 12-member Guardian Council, a clerical body appointed by Khamenei.
Geneive Abdo, an Iranian expert at U.S. think tank The Brookings Institution, said in an article published Friday by Brookings the ruling cleric has "no tolerance" for showmanship on the international stage.
Candidates last week debated foreign policy issues, holding to commitments to nuclear research despite international concerns. U.S. State Department spokeswoman Jen Psaki told reporters Thursday, before Iranians started to vote, there were concerns about the fairness of the elections given the clerical influence.
Abdo said Khamenei worked to diminish presidential powers during President Mahmoud Ahmadinejad's tenure.
"Based on the selection of this year's presidential contenders by the Guardian Council, he is determined that the next president will execute his orders on all matters," she said.