Iran's President Mahmoud Ahmadinejad has won 69 percent of the vote after over five million votes were counted in the presidential election on Friday, Iran's electoral chief said.
He made his comments at a news conference after ballots closed.
In the meantime, moderate former Prime Minister Mir Hossein Mousavi claimed victory in the hotly contested vote, while state news agency IRNA said incumbent Ahmadinejad had won.
The conflicting claims came even as crowds of voters were still queuing to cast their ballots long after balloting was officially over at 10:00 p.m. (1730 GMT).
Even as the tail-enders were still voting, Mousavi called a news conference in Tehran to claim victory.
"In line with the information we have received, I am the winner of this election by a substantial margin," Mousavi said. He said many people had not been able to cast their ballots even after voting was extended by four hours.
Only minutes earlier, close Mousavi aide Ali Akbar Mohatshemi-Pour told AFP his candidate had won 65 percent of the vote.
IRNA, however, announced that Ahmadinejad had won the election.
"Doctor Ahmadinejad, by getting a majority of the votes, has become the definite winner of the 10th presidential election," it said.
The interior ministry said preliminary results were expected around 4:00 a.m. on Saturday (2330 GMT Friday).
In line with the information we have received, I am the winner of this election by a substantial margin
Mir Hossein Mousavi
Iran's economic woes and its relationship with the outside world emerged as key issues during a feisty campaign marked by carnival-like street rallies and acrimonious candidate debates on prime-time television.
The election turned the spotlight on deep divisions in Iran after four years under Ahmadinejad, whose firebrand rhetoric further isolated the country from the West while at home he has come under fire over his economic policies.
Ahmadinejad, 52, is seeking a second four-year term, while the 67-year-old Mousavi is hoping to make a comeback after two decades in the political wilderness, pledging better ties with the outside world.
U.S. President Barack Obama, who has called for dialogue with Iran after three decades of severed ties, said on Friday he saw the "possibility of change."
The race also includes reformist former parliament speaker Mehdi Karroubi and the ex-head of the elite Revolutionary Guards Corps, Mohsen Rezai, but they are running far behind.
A bitterly fought campaign has generated intense excitement inside Iran and provoked strong interest around the world, with policymakers looking for signs of a change of approach by Tehran, whose ties with the West worsened under Ahmadinejad.
Divisions in Iran
Analysts have been reluctant to forecast a winner, suggesting the vote may mirror 2005 when the relatively unknown Ahmadinejad scored a stunning upset in a second-round runoff against heavyweight cleric Akbar Hashemi Rafsanjani.
A runoff will be held on June 19 if no single candidate emerges with 50 percent plus one vote.
Mohsen Esmaili, a member of the powerful Guardians Council which supervises the election, forecast turnout of at least 70 percent of the 46.2 million-strong electorate.
The election has highlighted divisions after 30 years of clerical rule in a country where 60 percent of the population was born after the revolution, and it is being keenly watched by the international community to determine future foreign policy.
Both front-runners promised a better future as they cast their ballots, with Ahmadinejad hailing the large crowds gathered at the polling stations, where men and women, many dressed in black chadors, formed separate lines.
Mousavi has pledged to improve relations with the outside world, although there are doubts of a real shift in nuclear policy as all strategic decisions are in the hands of the all-powerful Khamenei.
Obama is offering to open dialogue with a nation branded by his predecessor as part of an "axis of evil" but Iran remains locked in a standoff with the international community over its nuclear drive, which the West fears could be a cover for ambitions to build atomic weapons.