Iranian opposition leader Mirhossein Mousavi urged supporters to stage more protests over the re-election of hardline President Mahmoud Ahmadinejad, a direct challenge to the Islamic Republic's leadership, Reuters reported.
Mousavi made a veiled 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 on Sunday and gunfire was heard in the north of the city, a bastion of support for the reformist former prime minister.
But state radio said on Monday that the capital had been calm overnight for the first time since the June 12 election.
"Protesting against lies and fraud (in the election) is your right," Mousavi, who came a distant second to Ahmadinejad in the poll, said in a statement on his website late on Sunday.
"In your protests, continue to show restraint. I am expecting armed forces to avoid irreversible damage," he said.
Iranian state television said 10 people were killed and more than 100 others wounded 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 election which returned the anti-Western Ahmadinejad to power has provoked the most violent unrest since the 1979 Islamic Revolution which ousted the U.S.-backed shah.
The authorities have branded the protesters as "terrorists" and rioters. Tehran's police commander Azizullah Rajabzadeh said police would "confront all gatherings and unrest with all its strength," the official IRNA news agency reported.
U.S. President Barack Obama, in the forefront of diplomatic efforts to halt an Iranian nuclear programme the West fears could yield atomic weapons, has urged Iran to stop violence against protestors.
German Chancellor Angela Merkel said the authorities should conduct a recount. Ahmadinejad has rejected such calls as western interference.
The tensions in Iran, a major gas and oil producer, assumed broader significance on Sunday with Jean-Claude Trichet, president of the European Central Bank, telling French radio they had added to risks facing the world economy and underlined the need for strengthening the global financial system.
In pro-Mousavi districts of northern Tehran, supporters took to the rooftops after dusk to chant their defiance, witnesses said, an echo of tactics used in the 1979 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.
There were no immediate reports of casualties and the shooting appeared an attempt to break up unsanctioned protests.
State radio said on Monday morning: "Tehran last night witnessed the first night of calm and peace since the election."
Government restrictions prevent correspondents working for foreign media from attending protests to report. Iran ordered BBC correspondent, Jon Leyne, out of the country.
Pro-reform clerics meanwhile increased pressure on Iran's conservative leadership.
Mohammad Khatami, a Mousavi ally and a moderate former president, warned of "dangerous consequences" if the people were prevented from expressing their demands in peaceful ways.
His comments, carried by the semi-official Mehr news agency, were implicit criticism of Khamenei, who has backed a ban on protests and defended the outcome of the election.
State television said a daughter of former president Akbar Hashemi Rafsanjani, a rival of Ahmadinejad, had been released after being detained together with four other relatives during the Saturday rally in Tehran.
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.