By Parisa Hafezi and Fredrik Dahl
TEHRAN (Reuters) - Hardline President Mahmoud Ahmadinejad resoundingly won Iran's election, preliminary official results showed on Saturday, but his moderate challenger alleged irregularities and claimed victory for himself.
The level of the incumbent's support, nearly twice as many votes as former Prime Minister Mirhossein Mousavi with most ballots counted from Friday's poll, confounded widespread expectations that the race would at least go to a second round.
Ahmadinejad's re-election is likely to disappoint Western powers aiming to convince Iran to halt work they suspect is aimed at making bombs, and could further complicate efforts by U.S. President Barack Obama to reach out to Tehran.
"It doesn't augur well for an early and peaceful settlement of the nuclear dispute," said Mark Fitzpatrick at London's International Institute for Strategic Studies.
A bitterly fought campaign generated strong interest around the world and intense excitement inside Iran. It revealed deep divisions among establishment figures between those backing Ahmadinejad and those pushing for social and political change.
Ahmadinejad accused his opponents of trying to undermine the Islamic Republic through a policy of detente with the West. Mousavi hit back by saying the conservative president had humiliated Iranians with his "extremist" foreign policy.
Listing several complaints before official results were announced, Mousavi said late on Friday that many people had not been able to vote and that there was a lack of ballot papers.
He also accused authorities of blocking text messaging, with which his campaign tried to reach young, urban voters.
"I am the definite winner of this presidential election," Mousavi told a news conference.
But the state election commission said Ahmadinejad had won about 63.8 percent of the votes from the election in the world's fifth biggest oil exporter, with 38 million counted.
Mousavi had around 32.7 percent support, said the commission. Based on an Interior Ministry estimate of more than 80 percent turnout of Iran's 46 million eligible voters, he could not beat Ahmadinejad with the votes still to be counted.
Trita Parsi, president of the Washington-based National Iranian American Council, expressed disbelief at the wide margin in Ahmadinejad's favour. "It is difficult to feel comfortable that this occurred without any cheating," Parsi said.
Speaking in Washington before the results were released, U.S. President Barack Obama said his administration was excited about the debate taking place in Iran and he hoped it would help the two countries to engage "in new ways."
STREET SCUFFLES
Western capitals had hoped a victory for Mousavi could help ease tensions with the West, which is concerned about Tehran's nuclear plans, and improve chances of engagement with Obama, who has talked about a new start in ties with Tehran.
Now they will have to find a way to deal with Ahmadinejad's government if they want to make progress in defusing the dispute over Iran's nuclear programme.
The three-week election campaign was marked by mudslinging, with Ahmadinejad accusing his rivals of corruption. They said he was lying about the state of the economy.
It was unclear how Mousavi's supporters, who thronged the streets of Tehran nightly during the campaign, might react to an Ahmadinejad victory. U.S. strategic intelligence group Stratfor called the situation "potentially explosive," with a considerable risk of unrest.
Scuffles broke out early on Saturday between police and chanting Mousavi supporters in a Tehran square, a Reuters witness said. Police say they have increased security across the capital to prevent trouble. All gatherings have been banned until the publication of final results.
Shortly before voting ended, some Tehran residents said they were unable to make international phone calls and some Internet servers went down.
Ahmadinejad draws his bedrock support from rural areas and poorer big city neighbourhoods. Mousavi enjoys strong backing in wealthier urban centres, especially among women and the young.
Two other candidates attracted only tiny voter support.
Turnout was heavy after a campaign marked by heated debate over inflation -- officially 15 percent -- and unemployment.
Ahmadinejad, 52, won power four years ago, vowing to revive the values of the 1979 Islamic revolution. He has expanded the nuclear programme, rejecting Western charges that it is aimed at bomb-making, and stirred international outrage by denying the Holocaust and calling for Israel to be wiped from the map.
Mousavi, 67, rejects Western demands that Iran halt uranium enrichment but analysts say he would have brought a different approach to Iran-U.S. ties and talks on the nuclear issue. Ultimately, however, nuclear and foreign policy are determined by the country's supreme leader, Ayatollah Ali Khamenei.
The United States has had no ties with Iran since shortly after the revolution but Obama said in Washington the United States had "tried to send a clear message that we think there is the possibility of change" in relations.
(Additional reporting by Hossein Jaseb, Hashem Kalantari, Zahra Hosseinian in Tehran and Alistair Lyon in Beirut; Writing by Dominic Evans and Fredrik Dahl; Editing by Alistair Lyon)
