Iran has over 46 million registered voters. The President is elected by direct vote and he needs to secure a simple majority. In case no candidate is able to get a simple majority, another round of election is held between the two top contenders, and the person who wins the majority of votes becomes the President.
The presidential campaign this year has turned increasingly bitter, with both camps hurling accusations at each other.
During a recent televised debate, Ahmadinejad alleged that some Mousavi supporters, including sons of powerful Iranian cleric Akbar Hashemi Rafsanjani, had received financial privileges in the past.
An enraged Rafsanjani has now threatened to sue the Iranian President.
Ahmadinejad's opponents have targeted him on the rising inflation in the country and his controversial economic policies.
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