Within hours of the warning, several hundred protesters gathered in central Tehran, and police used tear gas and fired into the air to disperse them, news agencies reported.

“There is a massive, massive, massive police presence,” to head off any attempt to demonstrate, Associated Press quoted a witness in Tehran as saying.

The warning, on the Guards’ website, was issued despite an admission by Iran’s most senior panel of election monitors that the number of votes cast in 50 cities exceeded the actual number of voters, according to a state television report.

The discrepancies could affect some three million ballots. But the authorities insisted that the discrepancies did not violate Iranian law. The Guardian Council, charged with certifying the election, said it was not clear whether they would decisively change the result.

A Revolutionary Guards’s statement on Monday told protesters to “be prepared for a resolution and revolutionary confrontation with the Guards, Basij and other security forces and disciplinary forces” if they continued their protests, news reports said.

Opposition Leader Mir Hussein Moussavi has urged the protesters to continue their defiance, but he could face arrest for doing so.

“Moussavi’s calling for illegal protests and issuing provocative statements have been a source of recent unrests in Iran,” Ali Shahrokhi, head of Parliament’s judiciary committee, semi-official Fars news agency reported, according to Reuters.

Moussavi used a posting on his website Sunday night to call on own supporters to demonstrate peacefully despite warnings from Iran’s supreme leader Ayatollah Ali Khamenei that no protests of the vote would be allowed.

“Protesting to lies and fraud is your right,” Moussavi said.

In an apparent response, the Revolutionary Guards told demonstrators on Monday to “end the sabotage and rioting activities”, calling their protests a “conspiracy” against Iran.

Foreign Ministry spokesperson Hassan Qashqavi called the turnout a “brilliant gem which is shining on the peak of dignity of the Iranian nation”. He accused Western powers and news organisations of spreading unacceptable “anarchy and vandalism.”

On Sunday, the police detained five relatives of Ali Akbar Hashemi Rafsanjani, who openly supported Moussavi. The relatives, including Rafsanjani’s daughter, Faezeh Hashemi, were released after several hours.

In the network of Internet postings and Twitter messages, a powerful and vivid new image emerged: a video posted on several websites that showed a young woman, called Neda, her face covered in blood. Text posted with the video said she had been shot.

The website of reformist candidate Mehdi Karoubi referred to her as a martyr who did not “have a weapon in her soft hands or a grenade in her pocket but became a victim by thugs who are supported by a horrifying security apparatus”.