Supreme Leader Ayatollah Ali Khamenei urged calm on Friday and hinted of a crackdown if Iranians continue their protests of the country's disputed presidential election.

Ayatollah Ali Khamenei, Iran's supreme leader, addresses a crowd at Tehran University on Friday, making his first public statement since last week's disputed presidential election. (IRIB/Associated Press)"Today the Iranian nation needs calm," Khamenei said in an address at Tehran University, where he spoke for the first time regarding the results of the June 12 election, which showed hardline President Mahmoud Ahmadinejad winning by a 2-to-1 margin.

Khamenei has already approved the landslide victory, which has spurred allegations of vote-rigging and sent tens of thousands of supporters from both sides into the streets of Tehran in massive protests.

In his first response to a week of protests of the disputed election, Khamenei told Iranians the presidential win was a "definitive victory" and that continued street protests would not have an impact on the outcome.

"Some may imagine that street action will create political leverage against the system and force the authorities to give in to threats. No, this is wrong," he said.

Khamenei's address was broadcast live on state television Friday and showed a crowded hall with thousands of people gathered outside the building.

End 'extremist behaviour'

Many in the crowd were wearing green, the colour representing reformist leader Mir Hossein Mousavi, while others carried posters of Ahmadinejad and Khamenei. People draped in Iranian flags were visible and some carried placards with anti-Western slogans.

Tens of thousands of people gathered at Tehran University to hear Supreme Leader Ayatollah Ali Khamenei's address. (IRIB/Associated Press)The diverse crowd listened quietly to the speech, occasionally bursting into cheers.

"The result of the election comes from the ballot box, not from the street," he said, calling for the protests to end.

All politicians should shun "extremist behaviour," he said, referring to the rallies.

"I call on all to put an end to this method [protests]. … If they don't, they will be held responsible for the chaos and the consequences," he said, hinting at a possible crackdown on protesters.

So far the government has not stopped the protests with force despite an official ban on them.

Journalists working for foreign media have been restricted from first-hand reporting on the streets of Tehran.

In a taped interview with CBS News, U.S. President Barack Obama said he was concerned by the "tenor and tone" of Khamenei's comments, and said Iran's government should "recognize that the world is watching" and that how it deals with people will signal "what Iran is and is not."

Hours after Khamenei's speech, residents in Tehran climbed on top of their roofs and chanted, "God is great" and, "Death to the dictator."

There were no reports from eyewitnesses of any clashes following Khamenei's speech.

However, Mousavi's supporters have another mass rally planned for Saturday afternoon, and there has been no indication that it will be cancelled.

Khamenei assured the crowd there was no election fraud because the legal system would not have allowed it. Iranians will not be returning to the polls because there was a "definitive victory" in the election with an 11 million-vote margin, he said.

Khamenei repeated his support for Ahmadinejad and urged Iranians to do the same, calling the results of the election "your victory."

Mousavi, who was scheduled to attend the service with other candidates, has not yet commented on Khamenei's statements.

Khamenei praised Iranians for taking part in the election and called it a "a magnificent show of responsibility of the people to determine the fate of their own country."

'They do not know the Iranian nation'

If the Iranian people did not feel free they would not have cast their votes in such large numbers, Khamenei said.

He added the election demonstrates the country's religious democracy to the world and that it is Iran's enemies who are trying to target the legitimacy of the Islamic establishment by questioning the vote.

Khamenei repeated accusations that foreign countries are using official statements by government officials and websites to meddle in Iran's internal affairs and promote civil unrest.

"They do not know the Iranian nation. I strongly condemn such interference," Khamenei said.

Khamenei specifically blamed the U.S. and the U.K. for fanning the civil unrest in the country.

A man lies on the back of a taxi after being injured by gunfire at a rally supporting opposition presidential candidate Mir Hossein Mousavi on June 15. (Associated Press)British Prime Minister Gordon Brown said Friday that Iran's ambassador to London had been summoned to explain the comment, but a more junior diplomat ended up at a meeting with political director Mark Lyall Grant.

The European Union has unanimously condemned the violence being used against protesters in Iran. Foreign Affairs Minister Lawrence Cannon made similar statements on Thursday.

Western-based human rights organizations have said at least 500 people have been arrested, detained or disappeared since the election, including many prominent activists and politicians.

The mounting rallies have shown Mousavi has a strong backing to sustain his challenge to the election outcome.

But Khamenei left only a small window for opponents to change the results, saying complaints of the opposition candidates must be pursued within the established system.

Democracy and religious authority

The street protests have presented one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged from the Islamic Revolution.

Though Iran's ruling clerics still command deep public support, Mousavi's movement has some analysts suggesting Iranians may soon be questioning Khamenei's final authority in all state matters.

So far, protesters have focused on the results of the balloting rather than challenging the Islamic system of government. But a shift in anger toward Iran's non-elected theocracy could result in a showdown over the foundation of Iran's system of rule.

Khamenei had tried to strike a compromise earlier in the week, ordering the 12-member Guardian Council to conduct an investigation into the election results.

According to government officials, Ahmadinejad won 62.6 per cent of the vote, while Mousavi garnered 33.75 per cent in a contest that appeared to divide urban and rural voters.

The Guardian Council has said it will conduct a partial recount at disputed poll stations, and will examine the 646 complaints regarding the election results, including allegations of shortages of ballots, people on site trying to force citizens to vote for a particular candidate and expelling candidates' representatives from polling stations.

Mousavi has indicated he does not expect the review to be successful. He alleges the Guardian Council is not neutral and has already indicated it supports Ahmadinejad. He wants an independent investigation.

Mousavi and two other candidates who ran against Ahmadinejad will meet with the Guardian Council on Saturday to further discuss the complaints.

With files from The Associated Press