Iran's Supreme Leader Ayatollah Ali Khamenei has appealed for calm and attacked 'enemies' questioning the result of last week's presidential election.

In a rare public appearance, he said Iran's enemies were targeting the legitimacy of the Islamic establishment by disputing the outcome of the election.

'Today the Iranian nation needs calm,' Ayatollah Khamenei said in his first address to the nation since the upheaval began.

Tens of thousands of Iranians had gathered in and around Tehran University to hear Ayatollah Khamenei lead the Friday prayer sermon.

The Supreme Leader, Iran's ultimate authority, had earlier urged his compatriots to unite behind President Mahmoud Ahmadinejad, the declared winner of last Friday's poll.

But supporters of the runner-up, Mir-Hossein Mousavi, have so far ignored the call, holding huge unauthorised rallies.

Mr Mousavi's supporters had planned another demonstration on Saturday.

But it is understood that Mr Mousavi has no plans to call a rally on Saturday or Sunday.

Karoubi calls for election cancellation

Defeated presidential candidate Mehdi Karoubi has called the election result to be cancelled.

In an open letter which will be delivered to the Guardian Council, tomorrow, Mr Karoubi said: 'Accept the Iranian nation's will by cancelling the vote and guarantee the establishment's survival.'

Keeping up the pressure on the Islamic regime over the disputed vote, tens of thousands of supporters of Mr Mousavi carried out a demonstration in Tehran yesterday, the sixth straight day of protests in the capital.

The rally came as the electoral watchdog, the Guardian Council, announced it was investigating 646 complaints of poll violations from Mr Mousavi and the other two losing candidates.

The council also invited the trio to set out their grievances, with a decision by Sunday about any possible recount.

Mr Mousavi joined the crowds yesterday, most of whom were dressed in black as a mark of mourning for demonstrators killed in clashes during the protests, which have been unlicensed by the authorities.

The crowds dispersed quietly about five hours later.

Foreign journalists have been barred from attending all rallies or other events without express authority.

World powers have raised concern about the violence and widespread arrests, with EU leaders set to condemn the use of violence against protestors.

According to a draft statement Tehran will also be urged to investigate the claims of electoral fraud.