The BBC quoted Iranian state TV r as saying that Khamenei had told the guardian council, the clerical body that oversees elections, to examine the pro-reform candidate Mir Hossein Mousavi's claims of widespread rigging in Friday's poll.

The government declared the incumbent president, Mahmoud Ahmadinejad, to have won in a landslide victory.

Today's news represents a surprising turnaround for Khamenei, who had previously welcomed the results.

Mousavi has cancelled a rally planned for later today after being warned that militias responsible for policing it would be equipped with live ammunition.

The rally had earlier been banned by Iran's interior ministry, but it remained unclear whether protesters would take to the streets or not because many may be unaware that the demonstration had been cancelled, the BBC said.

A message broadcast on state radio said: "The interior ministry issued a statement and said no permission had been issued for a rally ... the holding of such a gathering would be illegal."

Ahmadinejad claimed the election results were fair and compared the protests that have taken place over the past three days to the passion shown by football fans after a game.

The news came as reports spread that dozens of leading members of the opposition had been detained, and that security forces had raided a dormitory at Tehran University, killing three people and injuring 15. The reports could not be confirmed.

Yesterday, Iranians protested against what many have called a "stolen" election.

The official result – 63% for Ahmadinejad and 34% for Mousavi, his main rival – means four more years for the president and an end to hopes for reform at home and perhaps for detente with the west.

By last night, all three contenders to the presidency, reformist and conservative, had raised serious doubts over the result.

Mousavi claimed that, on Friday night, he had been informed by the interior ministry that he had won the election convincingly.

Mousavi has also challenged the decision by Iran's supreme leader to endorse the official results.

He is reported to have met Khamenei and asked him to reconsider that endorsement.

In a letter to the guardian council, Mousavi is also reported to have said: "Fraud is evident and review and nullification is requested."

A spokesman for the politician told the BBC: "I have been asked to warn the world that this is a coup d'etat, and Mousavi has asked for the world's governments not to recognise a president created by a coup."

Ahmadinejad has rejected claims of fraud as a "psychological war" by the foreign media.

At a victory speech, he told tens of thousands of his supporters: "Some say the vote is disrupted, there has been fraud. Where are the irregularities?"

In a speech punctuated by roars of approval, he said: "Some people want democracy only for their own sake. Some want elections, freedom, a sound election. They recognise it only as long as the result favours them."

Last night, the US Vice-President Joe Biden said there was "some real doubt" over the result as the White House raised concern over "irregularities".

Published: 15 June 2009 11:16 GMT  | Last updated: 15 June 2009 11:16 GMT