"In the current sensitive situation ... the Guards will firmly confront in a revolutionary way rioters and those who violate the law," said a statement on the Guards' website.
The statement by the Guards, viewed as the most loyal guardians of the ruling clerical establishment, clearly signalled a crackdown on any fresh unrest over the re-election of hardline President Mahmoud Ahmadinejad.
Despite the warning, about 1,000 supporters of Mousavi gathered in a central Tehran square, a witness said.
"I was passing through Haft-e Tir square and I saw around 1,000 people there," the witness said.
Mousavi, who was officially beaten into second place by Ahmadinejad in an election which he says was rigged, called late on Sunday for fresh protests by his supporters.
"Protesting against lies and fraud (in the election) is your right," Mousavi said in a statement on his website.
The Revolutionary Guards said they would not hesitate to confront "illegal" protests by defeated presidential candidates, and warned the West to stop backing "rioters".
Supreme Leader Ayatollah Ali Khamenei met Ahmadinejad, parliamentary speaker Ali Larijani and judiciary chief Ayatollah Mahmoud Hashemi Shahroudi on Monday to discuss post-election developments, the ILNA news agency said. It did not elaborate.
Evoking the prospects of legal action against Mousavi, Ali Shahrokhi, head of parliament's judiciary committee, said his call for "illegal protests and issuing provocative statements" had been a source of unrest.
"Such criminal acts should be confronted firmly," he said, the semi-official Fars news agency reported. "The ground is paved to pursue Mousavi legally."
Iranian authorities have accused Western powers of supporting the widespread street protests and have not ruled out expulsions of some European ambassadors.
In Rome, the Italian Foreign Ministry said Italy was prepared to open its embassy in Tehran to wounded protesters in coordination with other European nations.
The move follows a Swedish initiative to look into whether European Union nations could put together a plan to take in and provide aid to demonstrators at their embassies in Iran, the ministry said.
Iranian state television said 10 people were killed and more than 100 wounded in demonstrations in Tehran on Saturday, which defied a warning from Supreme Leader Ayatollah Ali Khamenei.
"VANDALS" BLAMED
The office of Tehran's prosecutor general blamed the weekend deaths on "unknown vandals" who had opened fire on civilians and killed people on Saturday, Press TV, Iran's English-language television channel, quoted it as saying.
The tough warning by the Guards came after the capital had passed its most peaceful night since the election.
Young supporters of Mousavi urged people to carry black candles with green ribbons and encouraged motorists to turn on their headlights for two hours to demonstrate solidarity with victims of the violence, their website said.
The unrest in Iran, a major oil and gas producer, is the most widespread since the 1979 Islamic Revolution, which ousted the US-backed shah.
The authorities reject charges of fraud but a spokesman for Iran's top legislative body, which is looking into complaints by the defeated election candidates, conceded that the number of votes had surpassed eligible voters in some constituencies.
"Based on initial information, 50 towns had this problem," Guardian Council spokesman Abbasali Kadkhodai was quoted by state broadcaster IRIB as saying on Sunday evening.
He said this might be due to Iranians being able to vote wherever they want, as well as other factors. He said inspectors would look into the issue.
"However, the total votes in these constituencies do not exceed 3 million and consequently will not have any impact on the election," he said.
Iranians on social networking sites called for mourning for 'Neda', a young woman shot dead on Saturday. Footage of her death has been watched by thousands on the Internet and her image has become an icon of the protests.
But witnesses said security officials prevented her funeral from going ahead, blocking roads leading to a central Tehran mosque where the ceremony was to have taken place.
"Police were spraying paint on the cars of those who insisted on driving towards the mosque," said one witness.
Britain announced it was withdrawing the families of embassy staff in Iran because of the violence, which Iran continued to blame on the West - principally Britain and the United States.
"The promotion of anarchy and vandalism by Western powers and media is by no means acceptable," Iranian Foreign Ministry spokesman Hassan Qashqavi told a news conference.
US President Barack Obama, at 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 protesters.
Pro-reform clerics have increased pressure on Iran's conservative leadership, moderate former president Mohammad Khatami on Sunday warning of "dangerous consequences" if people were prevented from expressing their demands in peaceful ways.
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