Polling started in Iran at 8:00 am (0330GMT). It will last till 6:00 pm (1330 GMT) as per the government data. However, the polls may remain open until midnight depending on turnout among the 46-million-strong electorate. Results will be out within the 24 hours after voting ends.
This is Iran's 10th Presidential election since the 1979 revolution. While Ahmadinejad, 52, is battling for a second four-year term in office, Mousavi, 67, is seeking to make a comeback after two decades in the political wilderness. If former Tehran mayor Ahmadinejad is defeated, it will be the first time a sitting president has been ousted after a single term.