Sadegh Zibalalam, who teaches politics at the University of Tehran, told Al Jazeera that Khamenei had "come under some criticism for attaching himself too much to Ahmadinejad and supporting him uncritically and unreservedly".

Continuing protests

In depth



 Video: Iran supreme leader in 'power struggle'
 
Video: Iran's 'citizen journalists'
 Video: Iran steps up net censorship
 Video: Iranians go online to evade curbs
 Video: The struggle for power
 Video: Rival protests continue in Iran
 Video: One dead at Iran rally
 Video: Iranians rally in Europe
 Video: Poll result triggers Tehran protests

 Iran's Ayatollah under threat?
 
Iran curbs media after poll result
 Mousavi sees election hopes dashed
 Iran writer on poll result
 Mousavi's letter to the people
 Iran poll result 'harms US hopes'
 West concerned by Iran fraud claims
 What next for Iran?
 The Iranian political system
 Riz Khan: Iran's disputed election
 Inside Story: Iran election recount
 Inside Story: Iran's political future

 Your media: submit your clips of the protests to Al Jazeera 

"Against that background, it will be very difficult for him to make a U-turn and to declare the election null and void."

Keeping up the pressure on the authorities on Thursday, tens of thousands of protesters demonstrated for a sixth straight day.

The protests have continued despite the Guardian Council - a body of top Iranian clerics - saying it was investigating 646 complaints of poll violations submitted by Mir Hossein Mousavi and two other defeated presidential candidates.

The council, which oversees elections, also invited the trio of defeated candidates to set out their grievances on Saturday, with a decision about any possible recount of the vote expected on Sunday.

On Thursday, Mousavi addressed a huge rally of supporters in Tehran despite warnings from the Basij militia, which has been at the forefront of action against anti-government protests.

The gathering took place at the capital's Imam Khomeini Square and most were dressed in black to mourn for demonstrators killed in clashes earlier during the week.

Al Jazeera's Alireza Ronaghi, reporting from Tehran, said protesters from various walks of life had joined the rally "to get their votes back".

"People were sad, they were quiet, but they were determined. That's what we hear from the people who have been in the rally," he said.

"I have also talked to others, they say that the quiet mood ruling over all the marches have gone to a new height recently."

'Determined' Mousavi

Mousavi continues to call for a re-election and not a recount of the votes [GALLO/GETTY]
Mousavi told the crowd that a rally scheduled for Friday had been cancelled, and that his supporters should prepare for a major march on Saturday afternoon from Tehran's Revolution Square to Freedom Square.

Protest rallies have been staged in Tehran every day since the controversial outcome of the election was announced.

Zibalalam of the Tehran University said Mousavi had so far "been very determined and defiant".

"In every speech that he has made during the past five days, he has assured his followers that he is not going to back down, that he is going to stand firm, and he doesn't the accept the election results.

"He has been very, very determined and very dependable."

About 100 people gathered outside the United Nations building in Tehran earlier on Thursday urging the Guardian Council to take action over the disputed poll.

Officials have barred the foreign media from covering such "unauthorised" events.

The authorities have also shut down internet sites and mobile telephone text services in an attempt to bring the protests under control.

Despite these measures, violent scenes of police beating Mousavi supporters taken on mobile phones have been broadcast on news bulletins across the world, though the authenticity of such footage often cannot be verified.