There has been a huge turnout for Iran's closely-fought election as incumbent President Mahmoud Ahmadinejad seeks a second term in office.
"Voter turnout has been unprecedented," election commission chief Kamran Daneshjoo said, as long queues were reported at polling stations.
Polling has been extended by at least another four hours.
Mr Ahmadinejad faces a strong challenge from former PM Mir Hossein Mousavi in a campaign dominated by the economy.
The election is being watched closely around the world for signs of a possible shift in Tehran's attitude.
US President Barack Obama said he was "excited" about what was happening in Iran, which showed change was possible.
"Ultimately the election is for the Iranians to decide," he said.
"You're seeing people looking at new possibilities. And whoever ends up winning the election in Iran, the fact that there's been a robust debate hopefully will help advance our ability to engage them in new ways."
If no candidate gets 50% in the first round, the two front-runners will face a run-off vote.
There has been a surge of interest recently in Iran's presidential election, with unprecedented live television debates between the candidates and rallies attended by thousands.
Mr Daneshjoo has said the turnout could exceed the 1997 election, in which reformist Mohammad Khatami was elected. State-run Irna TV said more than five million people cast their vote in the first four hours of voting.
The country's Supreme Leader Ayatollah Ali Khamenei and both the leading candidates voted early in the day, calling on Iranians to exercise their right to choose the country's next president.
"I recommend them to just vote based on their own views and decisions," Ayatollah Khamenei said as he voted.
"God willing, the best and the most deserving person will be elected as the head of the executive body for a four-year period."
Mr Ahmadinejad thanked the people of Iran "for their goodness, for their greatness, for their selflessness, their sacrifices, and for their forgiveness".
Mr Mousavi said simply: "God willing, with the nationwide participation of the public, we will see better and more beautiful days."
Lets hope Iran gets some change, but if Ahmadinejad wins we can only hope the CIA gets him killed.