Iran's supreme leader has issued a stern warning that protests against the country's disputed presidential election results must end.
In his first public remarks after days of protests, Ayatollah Ali Khamenei said the outcome had to be decided at the ballot box, not on the street.
He said political leaders would be blamed for any violence.
US President Barack Obama said the Iranian government should be aware that "the world is watching".
Demonstrators calling for a new election earlier vowed to stage fresh protests on Saturday.
But the governor of Tehran province, Morteza Tamadon, has said no permission has been given for such a rally and he hoped it would not be held.
Chants of support
Addressing huge crowds at Tehran University, the ayatollah voiced support for President Mahmoud Ahmadinejad, saying the president's views on foreign affairs and social issues were close to his.
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Very robust words indeed from the Supreme Leader. Ayatollah Khamenei said the election results are fine and there's no way that any vote rigging could have happened in this nation.
That would be treacherous, he said. He has effectively endorsed Mahmoud Ahmadinejad's landslide win. He said it just isn't possible to have vote rigging here.
In the meantime, he has told Western nations "keep your nose out of our business, these are domestic affairs you are meddling in and you are responsible for the allegations of vote rigging to begin with".
Mr Ahmadinejad was among the thousands of people who packed the campus and surrounding streets, punctuating the ayatollah's speech with chants.
Responding to allegations of electoral fraud, the ayatollah insisted the Islamic Republic would not cheat.
"There is 11 million votes difference," the ayatollah said. "How can one rig 11 million votes?"
He appealed to candidates who had doubts about the election result to pursue any challenges through legal avenues.
BBC Tehran correspondent Jon Leyne says that Ayatollah Khamenei appears to have staked everything on this election result and Mr Ahmadinejad.
It all points to heavy crackdowns if the protests continue, our correspondent says.
In his highly anticipated address after Friday prayers, the ayatollah said despite differences of opinion among the presidential candidates, they were all trustworthy and loyal to the Islamic Republic.
He said the election was a "political earthquake" for Iran's enemies - singling out Britain as "the most evil of them" - whom he accused of trying to foment unrest in the country.
"Some of our enemies in different parts of the world intended to depict this absolute victory, this definitive victory, as a doubtful victory," the supreme leader said.
Some of our enemies... intended to depict this absolute victory, this definitive victory, as a doubtful victory
The UK government summoned the Iranian ambassador to protest against the ayatollah's remarks, although the embassy sent a more junior diplomat in his place.
In Washington, the House of Representatives voted 405-1 for a statement supporting democratic and fair elections, condemning the "ongoing violence" and the Iranian government's "suppression of independent electronic communications through interference with the internet and cell phones."
President Obama later told US broadcaster CBS: "I'm very concerned based on some of the tenor and tone of the statements that have been made, that the government of Iran recognise that the world is watching.
"And how they approach and deal with people who are, through peaceful means, trying to be heard will, I think, send a pretty clear signal to the international community about what Iran is and is not."
Amnesty International said it was "extremely disturbed" by the speech, saying that it indicated the "authorities' readiness to launch violent crackdowns if people continue to protest".
Amnesty says its reports suggest that up to 10 protesters have been killed in clashes with security forces and plain-clothed militias.
It was revising its earlier report of 15 dead but said that at least four students were still unaccounted for after an attack on a Tehran University dormitory.
Iran has seen repeated opposition rallies since the presidential result was declared last Saturday.
More than 100,000 people took to the streets of Tehran on Thursday in a "day of mourning" for eight protesters killed in the capital on Monday by members of the pro-government Basij volunteer militia.
UN High Commissioner for Human Rights Navi Pillay has expressed concern at the number of arrests of opposition supporters and urged the government to rein in the militias.
The official results gave Mr Ahmadinejad 63% of the vote against 34% for his main election rival, Mir Hossein Mousavi.
Defeated candidate Mehdi Karroubi has joined calls for the election to be declared void.
In an open letter to the electoral authorities, he wrote: "By deciding fairly to cancel the election and hold it again, you would be accepting the nation's will and guaranteeing the permanence of the system."
The Guardian Council - Iran's main electoral authority - has invited Mr Mousavi, Mr Karroubi and the other defeated candidate to discuss their objections on Saturday.
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