Iran's defeated opposition candidate Mir Hossein Mousavi has cancelled a big rally, amid growing unrest over last week's presidential poll, BBC reported.
The government had declared planned protests against the re-election of President Mahmoud Ahmadinejad illegal.
Mr Mousavi, a moderate, has called for the results to be annulled, alleging widespread fraud.
Mr Ahmadinejad says the poll was fair. Tens of thousands rallied in Tehran on Sunday to celebrate his victory.
'Death to the dictator'
There were clashes between the police and anti-Ahmadinejad protesters in several parts of the city for a second day on Sunday.
Police hit protesters with batons and charged them on motorbikes.
Mousavi supporters cried "death to the dictator" into the evening. Scores of people are reported to have been arrested.
There have been reports of tear gas being fired and rooms being searched at Tehran University.
The BBC's Jon Leyne in Tehran says the rapidly spreading protests are a challenge not just to the election result, not just to President Ahmadinejad, but to the supreme leader, Ayatollah Ali Khamenei himself.
That means it is, he says, a challenge to the whole basis of the Islamic Republic.
Mr Ahmadinejad dismissed the unrest as "passions after a soccer match".
International concern
Mr Mousavi's website carried a statement saying he had formally called on Iran's Guardian Council, which must certify the counting, to annul the election.
He added: "I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way."
But on Monday, the interior ministry said: "Some seditious elements had planned to hold a rally."
It added: "Any disrupter of public security would be dealt with according to the law."
A supporter of Mr Mousavi, Shahab Tabatabaei, told the Associated Press news agency the candidate had met Ayatollah Khamenei to press his call.
But the BBC's John Simpson in Tehran says the call is almost certain to fail.