Polling stations have closed across Iran, after 15 hours of voting.
Officials extended voting by several hours Friday to accommodate what they described as a large turnout in the country's presidential election. It is unclear when the results will be announced.
Millions of Iranians voted to choose a successor to President Mahmoud Ahmadinejad. Western news agencies reported long lines at some polling stations.
An Iranian election official told the French news agency turnout was likely to reach at least 70 percent.
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, who was barred from running by Iran's Guardian Council of clerics and jurists, had also urged his supporters not to boycott the election.
Five other candidates were 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 early Friday, 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 requests for 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.