* Supreme Leader tells Iranians to respect result
* Interior minister says Ahmadinejad gets 63 pct of vote
* Challenger Mousavi denounces result as "dangerous charade"
* Outcome seen as blow to nuclear diplomacy, ties with U.S.
* Rival supporters clash in Tehran streets
(Recasts with Supreme Leader, final result, clashes)
By Parisa Hafezi and Fredrik Dahl
TEHRAN, June 13 (Reuters) - Supreme Leader Ayatollah Ali Khamenei on Saturday told all Iranians to respect Mahmoud Ahmadinejad’s victory in a presidential election that his closest challenger described as a "dangerous charade".
Trouble erupted on the streets when Ahmadinejad partisans clashed with about 2,000 supporters of moderate former prime minister Mirhossein Mousavi who had been staging a protest against the result of Friday’s vote, a Reuters witness said.
The scale of Ahmadinejad’s triumph upset widespread expectations that the race would at least go to a second round, and his victory is unlikely to help unblock a standoff with the West over Iran’s nuclear programme.
Khamenei, Iran’s top authority, told defeated candidates and their supporters to avoid "provocative behaviour" as police with batons moved in to disperse stone-throwing Mousavi supporters, from Vanak Square in the capital. Some protesters were arrested and two men were carried away from the scene.
"The chosen and respected president is the president of all the Iranian nation and everyone, including yesterday’s competitors, must unanimously support and help him," Khamenei said in a statement read on state television.
Interior Minister Sadeq Mahsouli, an ally of the hardline Ahmadinejad, declared that the president had been re-elected to a second four-year term with 62.6 percent of the vote, against 33.7 percent for Mousavi, in a record 85 percent turnout.
Mousavi, a veteran of the 1979 Islamic revolution, protested against what he said were many obvious election violations.
"I’m warning I will not surrender to this dangerous charade. The result of such performance by some officials will jeopardise the pillars of the Islamic Republic and will establish tyranny," Mousavi said in a statement made available to Reuters.
He had been due to hold a news conference, but police at the building turned journalists away, saying it was cancelled.
Iranian and Western analysts abroad greeted the results with disbelief. They said Ahmadinejad’s re-election would disappoint Western powers aiming to convince Iran to halt work that 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 bitter election 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 rivals of undermining the Islamic Republic by advocating detente with the West. Mousavi said the president’s "extremist" foreign policy had humiliated Iranians.
On Friday night, before official results emerged, Mousavi had claimed to be the "definite winner". He said many people had been unable to vote and ballot papers were lacking.
He also accused authorities of blocking text messaging, with which his campaign tried to reach young, urban voters.
On Saturday, Iran’s students’ news agency ISNA quoted Tehran’s Deputy Prosecutor General Mahmoud Salarkia as saying 10 people had been detained for "agitating public opinion through websites and blogs by propagating untruthful reports".
Trita Parsi, president of the Washington-based National Iranian American Council, expressed astonishment at the wide margin in Ahmadinejad’s favour. "It is difficult to feel comfortable that this occurred without any cheating," he said.
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 of a new start if Tehran "unclenches its fist".
Now they must again deal with Ahmadinejad, who has refused talks with six world powers 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 economy. Inflation, officially put at 15 percent, and unemployment were core issues in the debate.
Scuffles broke out overnight between police and chanting Mousavi supporters in a Tehran square, a Reuters witness said. Police said they had boosted security across the capital. All gatherings were banned until final results are declared.
Ahmadinejad draws most of his 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 minimal support.
Ahmadinejad, 52, won power four years ago, vowing to revive the values of the Islamic revolution. He has expanded the nuclear programme, which Iran says is only for electricity generation, and stirred international outrage by denying the Holocaust and calling for Israel to be wiped off the map.
"If there was a shadow of hope for a change in Iran, the renewed choice of Ahmadinejad expresses more than anything the growing Iranian threat," Israel’s Deputy Foreign Minister Danny Ayalon said in a statement. "The international community must stop a nuclear Iran and Iranian terror immediately."
Mousavi, 67, rejects Western demands that Iran halt uranium enrichment, but argued for a different approach to Iran-U.S. ties and nuclear talks — although these are policy areas ultimately controlled by Supreme Leader Ayatollah Ali Khamenei.
The United States has had no ties with Iran, the world’s fifth biggest oil exporter, since shortly after the revolution. Obama said his country had "tried to send a clear message that we think there is the possibility of change". (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 and Kevin Liffey)