Iran was voting Friday in presidential elections that will determine whether the country will continue on the path of opening up to the West.
Fifty-six million Iranians are eligible to vote in the election, which is being seen as a referendum on incumbent Hassan Rowhani's battle to keep the country on a reformist course.
Some 63,500 polling stations opened at 8 am (0330 GMT) and there were long queues of voters in the capital, Tehran, and other parts of the country.
"Today is an important day and Iranians have their fate and that of their children in their hands," said Rowhani as he voted in Tehran. He said that voters should also keep in mind that the election outcome is extremely important for the country's foreign policy.
Rowhani's main rival of the four candidates is Ebrahim Raisi, an arch-conservative cleric and lawyer.
The other two candidates - Mostafa Hashemi-Taba, a reformer who had previously served as vice president under president Mohammad Khatami (1997-2005); and Mostafa Mirsalim, who served as culture minister between 1994 and 1997, but afterwards turned his back on politics - are not viewed as contenders.
While Rowhani may be the popular favourite, Raisi has the support of the state's powerful body of unelected clerics.
There is also speculation that Raisi could one day succeed the supreme leader, Ayatollah Ali Khamenei. As a politician he is largely a blank sheet, but in this election enjoys the complete support of the influential Islamic clergy.
Observers believe that the moderate president's best chances of a second term lie in a high voter turnout.
The election follows a heated campaign, in which Rowhani has accused his rival of "using religion" to score political points. The president has told voters that they face a choice between freedom and "women and men separated by walls."
Raisi has accused Rowhani of lies and incompetency, blaming him for high unemployment in the country.
The 56-year-old cleric said Rowhani's long-praised nuclear deal with six world powers - which set up international controls over Iran's nuclear programme in return for the end of 10 years of economic sanctions - had brought nothing for Iran.
Preliminary results are expected early Saturday. Interior Minister Rahman Fasli said the final outcome is expected Saturday evening or on Sunday.
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