At 3:33 p.m. ET: Iran's official news agency, IRNA, says President Mahmoud Adhmadinejad "has secured victory in Iran election," Reuters reports, although rival Mir Hossein Mousavi has also claimed victory.
Official results are not expected until tomorrow.
Voting was extended six hours, to midnight, because of the huge turnout.
Ahmadinejad 'set for Iran victory'
Mousavi claimed victory shortly after
the polls closed on Friday [AFP]
Mahmoud Ahmadinejad, Iran's incumbent president, has taken a commanding lead in his bid for re-election with more than two-thirds of ballot boxes counted, Iran's interior ministry has said.
Ahmadinejad is currently ahead with 65.2 per cent of the 28 million ballots counted against 31 per cent for Mir Hossein Mousavi, his main rival, according to results released early on Saturday.
IRNA, Iran's official news agency, announced that Ahmadinejad had won re-election.
"Doctor Ahmadinejad, by getting a majority of the votes, has become the definite winner of the 10th presidential election," it reported.
Al Jazeera's Teymoor Nabili, reporting from Tehran, said: "The state media have declared victory for Ahmadinejad and he not only won, he blew Mousavi away."
After the declaration, the president's supporters took to the streets of Tehran, waving Iranian flags and honking car horns.
"Where are the greens? In a mousehole," some of them said, referring to the campaign colours of Mousavi, whose supporters held mass rallies in recent weeks.
Mousavi had claimed victory just moments after polls closed on Friday.
"In line with the information we have received, I am the winner of this election by a substantial margin," he said. "We expect to celebrate with people soon."
But with the majority of votes counted according to Kamran Daneshjoo, chairman of the electoral commission at the interior ministry, the incumbent president had taken a seemingly unassailable lead.
Ahmadinejad had received 15,913,256 votes compared to 4,628,912 for Mousavi.
The two other candidates up for election, Mohsen Rezai, a former commander of Iran's Revolutionary Guard, and Mehdi Karroubi, an ex-parliament speaker - were set to finish a distant third and fourth with 470,549 votes and 212,855 votes respectively.
Al Jazeera's Nabili said that journalists following the elections have expressed surprise at the speed of vote counting.
"It does seem remarkably quick," he said. "But the explanation they are giving is that the counting has been going on throughout the day. They kept a running tally."
Latest reports show that 80 per cent of Iran's electorate voted in Friday's elections.
Mousavi alleged there had been irregularities in the voting, including a shortage of ballot papers.
He also accused the authorities of blocking text messaging, which his campaign has used to reach young voters.
Richard Dalton, a former British ambassador to Iran, told Al Jazeera: "There has been fraud alleged by the losing candidates in other Iranian elections and there has been good evidence produced that [it] actually happened.
"So one can't rule it out in this case. What is more important in the early stages is people's perceptions. If people perceive they were robbed that will stir up political passion in what is still a volatile country."
Scuffles broke out between police and chanting Mousavi supporters in a Tehran square early on Saturday, witnesses said. Police said they have increased security across the capital to prevent any trouble.
Never mind, Iranian woman are hot, hot, hot. Someday, maybe, I'll tell you my TWA story from Rome to Athens.