Shouts of 'Allahu Akbar' (God is Greatest) again reverberated around Tehran at nightfall, as Mousavi supporters took to the rooftops to chant their defiance of the authorities, a tactic used in the 1979 Islamic revolution.

'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.

Riot police and members of the Basij religious militia were deployed in force in downtown Tehran, and witnesses said 1,000 Mousavi supporters had gathered in Haft-e Tir square, despite the Guard's warning.

From his balcony, one witness saw a group chanting slogans being attacked by the Basij, who dragged the protesters out of a nearby house to which they had fled.

'The Basiji were really aggressive and swearing at me to go inside,' the witness said. 'I was scared they were going to break into my house too.'

Mr Mousavi, who was officially beaten into second place by Mr Ahmadinejad in the June 12 election which he says was rigged, called late on Sunday for fresh protests by his supporters 'against lies and fraud' in the election.

The unrest in Iran is the most widespread since the 1979 Islamic Revolution, which ousted the US-backed shah.

Supreme Leader Ayatollah Ali Khamenei met Mr 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.

Ali Shahrokhi, head of parliament's judiciary committee, said Mr Mousavi should be prosecuted for 'illegal protests and issuing provocative statements", the semi-official Fars news agency quoted him as saying. -- REUTERS