Incumbent Iranian President Mahmound Ahmadinejad has resoundingly won a hotly-contested election but his rival Mousavi has alleged widespread irregularities during Friday's polls.
Iranian election officials said on Saturday that incumbent President Mahmud Ahmedinejad won the election by a clear majority, getting twice as many votes as his closest challenger Mir Hossein Mousavi.
With almost all the votes counted, the Islamic Republic's election commission said Ahmadinejad had won more than 63 percent of votes against 34.7 percent for Mousavi. It put the voter turnout at around 80 percent of 46 million eligible voters.
Voting deadlines were extended several times on Friday to cope with long lines. Analysts had expected the vote to be a tight race between the hardliner Mahmud Ahmedinejad and the reformist Mousavi.
Ahmadinejad, seeking his second term in office, won most of his votes from the rural heartland.
Main challenger alleges voting irregularities
At a press conference, Mousavi said many people had not been able to vote and that there had been a lack of ballot papers.
He also accused the authorities of blocking text messaging, which his campaign has used to reach young, urban voters.
In Tehran, hundreds of Mousavi's supporters took to streets to protest the election outcome. There were reports of scuffles between demonstrators and police.
Observers have expressed concern over the possibility of post-election unrest. Police say they have increased security across the capital to prevent trouble.
Ahmadinejad assumed power four years ago on a pledge to revive the values of the 1979 Islamic revolution. He quickly drew international condemnation after refusing to halt Iran's nuclear programme, rejecting Western charges that it was aimed at building an atomic bomb.
He also denied the Holocaust and called for Israel to be wiped from the map.