Iran's official IRNA news agency said President Mahmoud Ahmadinejad won most votes in Friday's presidential election as his rival Mir Hossein Mousavi claimed that he had won a landslide victory in the election.
"Doctor Ahmadinejad, by winning most votes at the 10th presidential election, has secured his victory," IRNA said.
Earlier, opposition challenger Mousavi said that he was the "definite winner" in the election but he complained of voting irregularities.
"In line with the information we have received, I am the winner of this election by a substantial margin," Mousavi told a news conference in Tehran after the polling closed.
Millions of Iranians streamed to polling stations early on Friday, causing voting to be extended by four hours to 10 p.m. (1730 GMT) to allow more people to cast ballots, the Interior Ministry said. Voting was originally scheduled to close at 6 p.m.
Those still queuing when the polling stations closed were allowed to stay to vote. Officials say they expect turnout of 70 percent or more in the election.
Meanwhile Al Arabiya reported at around 7 p.m. local time that Iran had blocked seven websites linked to reformist candidates and some satellite channels, especially Persian ones.
Preliminary reports said news channel BBC Persia had been blocked and people were unable to SMS each other as services were blocked.
War of words
As the reformists claimed victory advisors to incumbent Ahmadinejad dismissed them as "psychological war" and said the outcome was impossible to predict.
Sadegh Kharazi, a senior backer of Mousavi, said surveys made by reformers showed that he was getting about 58-60 percent of the votes.
But an Ahmadinejad representative, Ali Asghar Zarei, said the incumbent was ahead with about the same level of support, the semi-official Mehr News Agency reported.
Mohammed Abtahi, head of reformist Mehdi Karoubi's campaign, said in an interview on Al Arabiya he expected Karoubi to win by a large margin.
Likewise Sadegh Kharazi, an ally of the former prime minister, told Al Arabiya that surveys showed his candidate had drawn 58 percent to 60 percent of the votes so far, enough votes to win outright in the first round.
"I can say that based on our surveys ... Mousavi is getting 58-60 pct of the vote and we are the winner," he said.
Meanwhile Ahmadinejad's supporters refuted such statements as "false" and intended as psychological warfare to affect voter turnout.
"These early statements are meant to influence votes," Ali Akbar Javanfekr, advisor to Ahmadinejad, told Al Arabiya.
"This is a psychological war they launched to influence voting," he said.
Unofficial voter count surveys showed Mousavi leading Ahmadinjad in eight provinces while other surveys showed Ahmadinejad's votes come mainly from the countryside.
Ali Shariati, advisor to former Iranian president Mohammed Khatami, told Al Arabiya that "all evidence indicates the progress of Mousavi."
Counting was underway in Qazween and Tabreez, two cities with Turkish majority, after polls closed with the results expected to tip in Mouavi's favor.
The semi-official Fars news agency said participation in Iranian villages reached 90 percent. However experts cast doubt on such huge turnout.
The Interior Ministry said voting was extended past the official 6 p.m. (1330 GMT) closing time to 8:30 p.m., due to an unprecedented turnout among the 46-million-strong electorate. Results are expected within 24 hours after voting ends.
"We are expecting our dear citizens to come forward and vote in the early hours," the interior ministry, which is in charge of organizing the election, said in a statement earlier in the morning.
Iran's Supreme Leader Ayatollah Ali Khamenei, one of the early ones to vote and who has the final say in the country's strategic issues, urged people to remain "calm" and thwart any attempt to create "tension in polling stations."
"Voting is a right and duty of the people. I urge people to arrive at the polling stations from early hours to vote and have their share in choosing the highest management of the country," he told reporters.
The country's 10th presidential election since the 1979 revolution is largely seen as a close two-horse race with passions running high after three weeks of mass rallies, stormy television debates and vicious mudslinging. Ahmadinejad accused his rivals of corruption and they said he was lying about the state of the economy.
Mousavi has complained that Ahmadinejad's foreign policy "undermined the dignity of Iran," and along with his fellow challengers accused the president of mismanaging the economy.
Iran, OPEC's second biggest oil exporter, is currently battling rampant inflation of 24 percent and a slump in earnings from crude oil as international prices have sunk from close to $147 last year to around $72.
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.
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.
Unprecedented voter turnout
Long queues formed at voting centers in the capital Tehran and elsewhere. Some people said they had waited for more than two hours to cast ballots.
"Turnout is unprecedented," said election commission chief Kamran Daneshjou.
A high turnout could indicate the participation of many pro-reformers who stayed away when Ahmadinejad won a surprise victory four years ago on a pledge to revive the values of the 1979 Islamic revolution.
The election has generated interest around the world with policymakers looking for signs of a change of approach by Tehran.
A victory for Mousavi might help ease tensions with the West, which is concerned about Tehran's nuclear ambitions, and improve chances of engagement with U.S. President Barak Obama who has talked about a new start in ties with Tehran.
For Iranians the election is a chance to pass judgment on Ahmadinejad's four years in office, particularly his management of the Islamic Republic's oil exporting economy, which is suffering from high inflation and unemployment.
If blacksmith's son and former Tehran mayor Ahmadinejad is defeated, it will be the first time a sitting president has been ousted after a single term.
Preliminary results are expected early on Saturday.
If none of the candidates win 50 percent of the votes, a run-off will be held on June 19 between the two front-runners.