Millions of Iranians are voting Friday to choose a new president and officials have extended voting hours twice in response to what they describe as a large turnout.
Iran's state run news agencies said the polls would stay open an additional three hours with the possibility of additional extensions.
Analysts say the high interest in the carefully orchestrated campaign may be due to the candidacy of moderate cleric Hassan Rowhani. Iran's former nuclear negotiator picked up the endorsements of leading reformists.
Former president Ali Akbar Hashemi Rafsanjani, another reformist leader, has also urged his supporters not to boycott the election.
Five other candidates are also bidding to replace current President Mahmoud Ahmadinejad. Most, including current chief nuclear negotiator Saeed Jalili and Tehran Mayor Mohammad Bagher Qalibaf, are considered hardliners who are loyal to the country's supreme leader, Ayatollah Ali Khamenei.
The supreme leader cast his vote in Tehran, telling U.S. officials who have been critical of the election, "the hell with you."
Mr. Khamenei had been calling on Iranians to vote in large numbers.
The election winner will be faced with an economy struggling with high unemployment and inflation, crippled by international sanctions imposed over Iran's disputed nuclear program.
While some candidates favor improved ties with the international community, major policy decisions rest with the supreme leader.
To win, a candidate must get 50 percent of the vote. If no one succeeds after the initial vote, a runoff election will be scheduled a week later.
Iran sealed most of its borders, rounded up dissidents and detained some journalists. Most foreign news organizations say their attempts to get visas to cover the election were unheeded by the government.
One candidate seen as reformist, Mohammad Reza Aref, dropped out this week and then announced his support for Rowhani. Another candidate, Gholam Ali Haddad Adel, dropped out of contention Monday saying he wanted to boost the chances of his fellow conservatives.
Two-term President Mahmoud Ahmadinejad is constitutionally barred from a third consecutive term.