US President Barack Obama has strongly condemned the "unjust actions" of Iran in clamping down on election protests.
He said he respected Iran's sovereignty and it was "patently false" of Iran to say the West was fomenting the unrest.
UN chief Ban Ki-moon had called on Iran to respect the "will of its people" after the disputed presidential poll but Tehran accused him of "meddling".
Earlier, the opposition was told by Iran's Guardian Council the 12 June election would not be annulled.
But Iran's Supreme Leader Ayatollah Ali Khamenei later agreed to extend by five days the amount of time allowed to examine complaints of electoral fraud.
Kim Ghattas BBC News, Washington DC
President Barack Obama has taken it up a notch while heeding the advice of Democrats and Iran experts who have been talking to the White House: "Start using more forceful language against the violence but avoid taking sides in Iran's power struggle".
So President Obama said he strongly condemned the unjust violence, that he was appalled and outraged by it, that he was struck by the courage of the demonstrators. But he added it was important to let the Iranian people know they were not alone in this process and that those who stand up for justice are always on the right side of history.
He didn't mention the name of a single Iranian leader and was careful to insist again that Washington respected Iran's sovereignty. And without withdrawing the offer of talks, he signalled that the violence would impact that dialogue - Mr Obama said that how Iran's leaders handle the dissent would shape Iran's future and its relations with the international community.
In the 11 days since the election result, which saw incumbent President Mahmoud Ahmadinejad returned with 63% of the vote, opposition supporters have clashed with police on the streets of the capital Tehran.
Mr Obama said: "The United States and the international community have been appalled and outraged by the threats, beatings, and imprisonments of the last few days.
"I strongly condemn these unjust actions, and I join with the American people in mourning each and every innocent life that is lost."
He said: "The United States respects the sovereignty of the Islamic Republic of Iran, and is not at all interfering in Iran's affairs. But we must also bear witness to the courage and dignity of the Iranian people, and to a remarkable opening within Iranian society."
Mr Obama said of the allegations of meddling: "This tired strategy of using old tensions to scapegoat other countries won't work anymore in Iran.
"This is not about the United States and the West. This is about the people of Iran, and the future that they - and only they - will choose."
Referring to the recent clampdown on the foreign media in Iran, Mr Obama said: "In 2009 no iron fist is strong enough to shut off the world from bearing witness to the peaceful pursuit of justice.
"Despite the Iranian government's efforts to expel journalists and isolate itself, powerful images and poignant words have made their way to us through cell phones and computers, and so we have watched what the Iranian people are doing."
"No iron fist is strong enough to shut out the world from bearing witness"
Earlier Mr Ban had urged the authorities in Iran to respect fundamental civil rights, "especially the freedom of assembly and expression", and end arrests.
However, Iranian foreign ministry spokesman Hassan Ghashghavi said: "These stances are an evident contradiction of the UN secretary general's duties, international law and are an apparent meddling in Iran's internal affairs."
He said the UN secretary general had "damaged his credibility" in the eyes of "independent" countries by "ignorantly following some domineering powers which have a long record of uncalled-for interference in other countries' internal affairs and colonisation".
On Tuesday, the country's legislative body, the Guardian Council, said there had been no major polling irregularities and the result would stand.
Guardian Council spokesman Abbas Ali Kadkhoda'i said there had been "no major fraud or breach in the election".
However, opposition supporters continued to call for the elections to be re-run, amid claims of vote tampering.
Among them was opposition candidate Mehdi Karoubi, who urged Iranians to mourn for dead protesters on Thursday.
We want a very good relationship with the Iranians, we also respect the fact that it's for the Iranian people themselves to choose who their government is
Gordon Brown British leader
His call echoed an earlier one from cleric Grand Ayatollah Hossein Ali Montazeri for three days of national mourning for those killed in the street protests.
The protests in the last 24 hours are smaller than they have been over the past 10 days, amid the strong security presence on the streets.
BBC Middle East editor Jeremy Bowen said the protesters were talking about finding other ways to show their opposition, including strikes or civil disobedience.
A spokesman for the US government said it "would not endorse" general strikes, but he added: "We've seen the beginnings of change in Iran."
In a fresh diplomatic move, Britain is expelling two Iranian diplomats in response to Tehran's decision to order two UK diplomats to leave Iran following allegations UK Prime Minister Gordon Brown called "absolutely without foundation".
Mr Brown told the BBC: "We want a very good relationship with the Iranians, we also respect the fact that it's for the Iranian people themselves to choose who their government is.
"But when there is a sign of repression or where there is violence that's affecting ordinary people in the streets, we have a duty to speak out and to say we want Iran to be part of the world, we don't want Iran to be isolated from the world."
The Iranian parliament's national security and foreign policy commission responded to the expulsion of the diplomats in London by reconsidering ties with Britain.
The meeting was attended by Foreign Minister Manuchehr Mottaki and the Iranian state broadcaster said certain decisions were made in the meeting that would be announced in due time.
On Friday, Ayatollah Ali Khamenei banned protests, prompting street violence in which at least 10 people died.
Severe reporting restrictions placed on the BBC and other foreign media in Iran mean protest reports cannot be verified independently.
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