Iran's Supreme Leader has ordered an investigation into the disputed re-election of President Ahmadinejad, reversing his earlier decree that the result was fair.

Ayatollah Ali Khamenei's intervention came as Iran braced itself for a third day of street protests by reformists, who claim that official voting figures giving Mr Ahmadinejad 63 per cent in last Friday's election were rigged, and a "charade".

Ayatollah Khamenei, who has the final say on all strategic matters, had described the result as fair and urged the country to unite behind Mr Ahmadinejad.

Today it emerged, however, that he had met the defeated opposition candidate Mir Hossein Mousavi last night and ordered the powerful Guardians Council to examine his allegations of vote-rigging.

"Naturally in this election, complaints should be followed through legal channels," Ayatollah Khamenei told Mr Mousavi, according to Iranian state television. "It is necessary you follow the issue calmly."

A clampdown on the protests escalated today when the authorities attempted to ban a nationwide march by Mousavi supporters.

Mr Mousavi's campaign website said that he will appear at the rally today despite the ban, and issue a plea for calm. The Interior Ministry warned that if Mr Mousavi did go ahead with his rally despite the ban, "the consequences of such behaviour will be directed at Mousavi", according to the IRNA state news agency.

Mr Mousavi has also threatened to embarrass the authorities by holding a sit-in protest at the mausoleum of the late Ayatollah Ruhollah Khomeini, the deeply revered founder of the 1979 Islamic Revolution, daring them to risk clashes at the hallowed site.

Police arrested 170 activists at the weekend, including the brother of former president Mohammad Khatami and a number of reformist leaders.

There have been unconfirmed reports of unrest breaking out in other cities across Iran. Opposition activists said that a student protester was killed early today during clashes in Shiraz, in southern Iran. If confirmed it would be the first fatality since the demonstrations began.

Dozens more protesters were arrested overnight, as police and militia stormed the campus at Tehran's biggest university after about 3,000 students at dormitories of Tehran University began a demonstration, chanting "Death to the dictator".

Clashes developed as students threw rocks and petrol bombs at the police, who fought back with teargas and plastic bullets, according to Akbar, a 25-year-old student who witnessed the fighting.

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