Mousavi also issued an oblique appeal to the security forces to show restraint in handling demonstrations — a move likely to be viewed with deep suspicion by a conservative leadership that has vowed to use force wherever necessary to quell opposition.
Helicopters buzzed through the evening sky over Tehran and gunfire was heard in the north of the city, a bastion of support for the reformist former prime minister.
“Protesting against lies and fraud (in the election) is your right,” Mousavi said in a statement on his website.
“In your protests, continue to show restraint. I am expecting armed forces to avoid irreversible damage,” he added.
Iran state television said 10 people were killed and more than 100 others injured in protests in Tehran on Saturday held in defiance of a warning from Supreme Leader Ayatollah Ali Khamenei. A separate report put the number of deaths at 13.
Mousavi said the mass arrest of his supporters, “will create a rift between society and the country’s armed forces”.
A product of the Islamic establishment himself, Mousavi said on Saturday he was not questioning the fundaments of the Islamic Republic but sought to renew it and purge it of what he called deceit and lies.
The dispute over the June 12 election which returned to power hardline anti-Western President Mahmoud Ahmadinejad has sparked the most violent unrest since the Islamic Revolution which ousted the U.S.-backed shah in 1979.
The authorities in Iran, a major gas and oil producer, have dismissed the protesters as “terrorists” and rioters, an indication of their determination to crack down hard on demonstrations.
Tehran’s police commander Azizullah Rajabzadeh said police would “confront all gatherings and unrest with all its strength,” the official IRNA news agency reported.
In pro-Mousavi districts of northern Tehran, supporters took to the rooftops after dusk to chant their defiance, an echo of tactics used in the 1979 Islamic revolution.
“I heard repeated shootings while people were chanting Allahu Akbar (God is greatest) in Niavaran area,” said a witness, who asked not to be named.
Another witness heard shooting in Zaferaniyeh district in the north of the capital. There were no immediate reports of casualties and the shooting appeared an attempt by the authorities to break up unsanctioned protests.
Government restrictions prevent correspondents working for foreign media from attending protests to report.
As authorities fulminated against protesters backing Mousavi, moderate former President Mohammad Khatami signalled increased opposition among pro-reform clerics to Iran’s conservative leadership.
“Preventing people from expressing their demands through civil ways will have dangerous consequences,” Khatami, a Mousavi ally, said in a statement quoted by the semi-official Mehr news agency.
His comment, implying criticism of Khamenei who has backed a ban on protests and defended the outcome of the election, found an echo with Grand Ayatollah Hossein Ali Montazeri, the most senior dissident cleric.
“Resisting people’s demand is religiously prohibited,” said Montazeri, an architect of the Islamic revolution who fell out with the present leadership and was under house arrest for some years.
Mousavi, who came second to Ahmadinejad in the poll and whose followers have spearheaded protests, says the election was rigged and must be annulled.
An analysis of official statistics from Iran’s Interior Ministry by Britain’s Chatham House think-tank suggested that in the conservative Mazandaran and Yazd provinces, turnout was more than 100 percent.
It said that in a third of all provinces, official results would have required Ahmadinejad to take all former conservative, centrist and all new voters, and up to 44 percent of reformist voters, “despite a decade of conflict between these two groups.”
The authorities reject charges of election fraud. But the highest legislative body has said it is ready to recount a random 10 percent of votes cast.
Khatami was sceptical. “Referring the dispute to a body which has not been impartial regarding the vote, is not a solution,” he said in a statement, Mehr reported.
