Iranians poured into polling stations Friday to vote in a tight presidential race with incumbent hardliner Mahmoud Ahmadinejad facing a tough challenge from moderate ex-premier Mir Hossein Mousavi.
Supreme leader Ayatollah Ali Khamenei urged Iranians to remain calm after an unusually impassioned election campaign marked by raucous street demonstrations and acrimonious mudslinging among candidates on prime-time television.
The election has turned the spotlight on deep differences in the Islamic republic after four years under Ahmadinejad, whose fiery rhetoric further isolated Iran from the West while at home he has come under fire over the country's dire economic crisis.
Election officials have been predicting record turnout among the country's 46.2 million voters, half of whom were born after the 1979 Islamic revolution that toppled the U.S.-backed shah.
"There have been a lot of people from the very beginning," election commission chief Kamran Daneshjoo told AFP. "The participation is unbelievable to us too. The winner is the Islamic republic and the people."
Ahmadinejad, 52, is battling for a second four-year term in office, while the 67-year-old Mousavi is seeking to make a comeback after two decades in the political wilderness.
Two other candidates, reformist former parliament speaker Mehdi Karroubi and the ex-head of the elite Revolutionary Guards Corps, Mohsen Rezai, are running far behind.
Analysts have been reluctant to forecast a winner, suggesting the vote may mirror 2005 when the relatively unknown Ahmadinejad scored a stunning upset in a second-round runoff against heavyweight cleric Akbar Hashemi Rafsanjani.
Daneshjoo said polling could be extended beyond the 6:00 pm (1330 GMT) closing time depending on turnout.
The three-week election campaign highlighted a glaring internal divide, with towns and villages passionately backing Ahmadinejad and young men and women in big cities throwing their weight behind Mousavi.