The moderate Iranian leader who says that he was robbed of victory in last week’s presidential election faces a fateful choice today: support the regime or be cast out.

Ayatollah Ali Khamenei, Iran’s Supreme Leader, has told Mir Hossein Mousavi to stand beside him as he uses Friday prayers at Tehran University to call for national unity. An army of Basiji — Islamic volunteer militiamen — is also expected to be bussed in to support the Supreme Leader.

The demand was made at a meeting this week with representatives of all three candidates who claim that the poll was rigged, and it puts Mr Mousavi on the spot. He has become the figurehead of a popular movement that is mounting huge demonstrations daily against the “theft” of last Friday’s election by President Ahmadinejad, the ayatollah’s protégé.

Mr Mousavi, 67, is a creature of the political Establishment — a former revolutionary and prime minister who would like to liberalise Iranian politics but has never challenged the system in the way his followers are doing. It was unclear last night what he would do or even whether the protests would die away if he backed down. Yesterday tens of thousands of demonstrators packed into the Imam Khomeini Square in Tehran — named after the founder of the Islamic Republic — for another massive rally, this one to mourn protesters killed in Monday’s clashes with pro-government militias.

Men, women and children from all backgrounds came dressed in black, with green wristbands, the colour of the Mousavi campaign, and staged a two-hour vigil. Some held banners bearing the names or pictures of the dead, and placards proclaiming “My martyred brother, we will get back your vote” or “We have not had people killed to compromise and accept a doctored ballot box”.

There was near silence until Mr Mousavi arrived with his wife, Zahra Rahnavard, and the throng broke into chants of “Mousavi! We support you!” With almost ecstatic fervour, the traditional lament for Shia Islam’s most important martyr, “Ya Hussein! was answered by the defeated presidential candidate’s forenames “Mir Hossein!

Mr Mousavi was accompanied by Mehdi Karoubi, another of the defeated candidates. He attempted to speak from the top of a car using a loudhailer, but few could hear him.

As the protesters departed just before dusk they left behind little shrines of black candles placed around photographs of the dead. “It encourages us to follow their path, fighting for the vote,” one woman said.

It was the sixth day of demonstrations and first to be held in the poorer south-central district, an apparent effort to dispel notions that the protesters are middle-class or pro-Western.

The Guardian Council, a body of 12 senior clerics whom the Supreme Leader has asked to investigate claims of electoral fraud, said it had received 646 complaints of irregularities. It has invited Mr Mousavi and the other two candidates challenging Mr Ahmadinejad — Mr Karoubi and Mohsen Rezai — to set out their grievances tomorrow and will decide on Sunday whether to order a recount. Nobody believes that it will. The Supreme Leader controls the council and is widely believed to have asked it to investigate as a way to play for time.

Mr Ahmadinejad defended the vote, telling a Cabinet meeting yesterday that “the ideals of the Islamic Revolution were the winners”.

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