Iranian President Mahmoud Ahmadinejad is seen during a military parade to commemorate Army Day, in front of the mausoleum of the late revolutionary founder Ayatollah Khomeini, just outside Tehran, Iran, Thursday, April 17, 2008. (AP Photo/Vahid Salemi)
State media announced early Friday that more than 45,000 polling stations across the country had opened.
Voter turnout is expected to be high during the 10 hours of voting.
The election is seen as a referendum on Ahmadinejad. His top opponent, Mir Hossein Mousavi, has energized many youth who want fewer hard-line Islamic restrictions and better international relations.
There are about 46 million eligible voters in Iran, a country of 70 million people.