Güncelleme Tarihi:
An aide to defeated presidential candidate Mehdi Karroubi told AFP the rally will be held in Tehran at 4:00 pm (1130 GMT).
The rally comes as Karroubi and the two other defeated candidates, former premier Mir Hossein Mousavi and ex-Revolutionary Guards' chief Mohsen Rezai, meet officials to discuss alleged electoral violations.
After supreme leader Ayatollah Ali Khamenei on Friday banned marches and warned that candidates would be held responsible for any violence, Tehran authorities denied permission for the march, organised by Karroubis campaign and a reformist group, Combattant Clerics Assembly.
Late on Friday, witnesses reported that many members of the hardline Basij militia had deployed in Tehran streets, for the first time in full uniform and helmets, carrying clubs and some of them Kalashnikov rifles. However, they had withdrawn from their positions on Saturday morning.
In demanding an end to protests, Khamenei warned that otherwise there could be further bloodshed beyond the seven deaths reported by state radio. Amnesty International said on Friday it had information of up to 10 deaths.
Beyond Khamenei's general warning, Mousavi was singled out by the head of Iran's security council on Saturday for a specific one.
"Your national duty tells you to refrain from provoking illegal gatherings," Abbas Mohtaj, who is also deputy interior minister, said in a letter to him. "Should you provoke and call for these illegal rallies you will be responsible for the consequences," he said.
Iran's capital has been rocked by daily demonstrations since the disputed re-election of President Mahmoud Ahmadinejad on June 12 drew claims from leading rival and former premier Mousavi of massive vote fraud.
Siding with Ahmadinejad in his first public appearance since the vote, Khamenei ruled out major fraud in the poll. "The people have chosen whom they wanted," Khamenei said in a prayer sermon on Friday, referring to Ahmadinejad.
"I see some people more suitable for serving the country than others but the people made their choice," he said to cheers from tens of thousands of faithful, who included Ahmadinejad.
"World is watching"
Afterwards, Obama warned Iran that the "world is watching" its actions. "I'm very concerned based on some of the tenor and tone of the statements that have been made that the government of Iran recognise that the world is watching," Obama said on US television on Friday.
"And how they approach and deal with people who are, through peaceful means, trying to be heard will, I think, send a pretty clear signal to the international community about what Iran is and is not."
Obama also attempted to debunk claims by some in the Iranian leadership that demonstrators were acting at the behest of the United States, which has had a history of antagonism with Iran since the 1979 Islamic revolution.
Senior US officials earlier stressed that Washington was making strenuous efforts to avoid being drawn into the crisis in a way that could be used by the government against the demonstrators.
"The more the United States looks like they are going to interfere, the more it is going to be detrimental," said one official on condition of anonymity. "This is not about us."
Despite assurances by top officials that Washington would not inject itself into the crisis, both houses of the US Congress voted to condemn violence against demonstrators by the Iranian government.
A House of Representatives resolution, adopted by a vote of 405-1, expressed "its support for all Iranian citizens who embrace the values of freedom, human rights, civil liberties and rule of law."
Mousavi, Karroubi and Rezai invited
Karroubi has become the second losing candidate to demand a new election, in a letter to the electoral watchdog the Guardians' Council.
Mousavi has repeatedly demanded a re-run of the poll, denouncing the election as a "shameful fraud."
But Khamenei said there could be no doubting Ahmadinejad's re-election to a second four-year term, despite the 646 alleged poll violations registered by the three defeated candidates with the Guardians' Council. "The legal mechanisms in our country do not allow cheating. How can one cheat with a margin of 11 million votes?"
Mousavi, Karroubi and Rezai have been invited to set out their grievances before the Guardians' Council election watchdog on Saturday, with a response expected on Sunday.
In addition to the United States, other world powers and entities have also expressed concern about the post-election violence and widespread arrests, with EU leaders, the UN human rights body and Amnesty International urging Iran to respect the right to protest.
In the face of the regimes biggest crisis since the 1979 revolution overthrew the pro-Western shah, Iran's Islamic rulers have repeatedly lashed out at "meddling" by foreign powers. Khamenei renewed the charge on Friday, singling out Britain.
"Today, top diplomats of several Western countries who talked to us so far within diplomatic formalities are showing their real face and most of all, the British government," he said.
Foreign Secretary David Miliband insisted he would not allow Khamenei to turn the Tehran protests into a "battle" between Britain and Iran.