US President Barack Obama has told Iran that "the world is watching", after its supreme leader demanded an end to post-election demonstrations, BBC reported.
Ayatollah Ali Khamenei said protest leaders would be responsible for any "bloodshed" if the rallies continued.
The ayatollah backs President Mahmoud Ahmadinejad's re-election, but other candidates are contesting the results.
More rallies are expected, and the powerful Guardian Council is due to meet the president's rivals.
The council has invited presidential candidates Mir Hossein Mousavi, Mehdi Karroubi and Mohsen Rezai to discuss more than 600 objections they have filed complaining about the poll.
Mr Mousavi, the main rival to Mr Ahmadinejad, has claimed that the ballot was rigged and wants a re-run.
Praise for protesters
President Obama has previously said he did not want to be seen to be meddling in Iran's affairs.
But he told broadcaster CBS on Friday: "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."
Earlier, White House spokesman Robert Gibbs had said the protests were "extraordinary" and "courageous".
In Washington, the House of Representatives voted 405-1 in favour of a statement supporting democratic and fair elections.
The statement condemned the "ongoing violence" and the Iranian government's "suppression of independent electronic communications through interference with the internet and cell phones".
Iran has seen repeated opposition rallies since the presidential result was declared a week ago.
The human-rights group Amnesty International says it believes about 10 people have been killed.
The official results gave President Ahmadinejad 63% of the vote against 34% for Mr Mousavi.
The BBC's Jonathan Beale in Washington says President Obama continues to tread a fine line in commenting on Iran - anxious not to be seen as interfering nor to fuel anti-American sentiment.
Although his comment in the CBS interview could be seen as his strongest warning so far, many Republicans still believe he has been been far too cautious.
'Political earthquake'
In his speech during Friday prayers, Ayatollah Khamenei criticised Western governments for their reaction to the re-election of President Ahmadinejad.
Ayatollah Khamenei hit out at what he called the arrogant powers, and media leaders in the US and some European countries who, he said, had shown their true faces.
He said the election was a "political earthquake" for Iran's enemies - singling out the UK as "the most evil of them" and accusing the British 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.
In a stern warning to protesters, he called on them to stop and said their political leaders would be blamed for any violence.
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?"
Correspondents say the tone points to heavy crackdowns if the protests continue.