Tehran is braced for renewed violence tomorrow as the regime’s opponents attempt to stage fresh demonstrations to mark the tenth anniversary of the student uprising of 1999.

The opposition sees the anniversary as a chance to show that it has not been crushed by the regime’s use of overwhelming force, and the demonstrators are planning to thwart the security forces by gathering in eight or nine places before converging on Enghelab (Revolution) Square. Demonstrations are also planned in several other cities.

However, the authorities are going to great lengths to stop them. They have taken advantage of the dust storms that have smothered the capital this week to close universities, offices and businesses, and to encourage people to leave the city. Residents said that half the city has decamped northwards across the mountains to the Caspian Sea.

The text messaging system has been shut down. The Basij volunteer militia is on high alert, and opponents have no doubt that the security forces will resort to violence if necessary. “I’m scared,” said one. “To go out on the streets in the present conditions is a bit suicidal.”

On Tuesday night President Ahmadinejad used his first nationally televised speech since his hotly disputed re-election to send a slightly more emollient message to the opposition.

He insisted that the ballot on June 12 was “the cleanest and healthiest election in the world”. He said that it had “doubled the dignity of the Iranian nation”, and marked the beginning of a new era for the country. He dismissed charges of electoral fraud, declaring: “No fault was discovered. The whole nation understood this.”

But he also promised a greater focus on the economy and unemployment during his second term. “We must respect people’s feelings, especially the youth,” he said, and hinted that the reviled ‘morality police’ could be reined in: “Cultural issues should be dealt with through cultural channels, and I am against security confrontations.”

As Mr Ahmadinejad spoke a moth fluttered around his head. Iranian bloggers joked yesterday that it would soon be arrested and forced to confess.

Supporters of Mir Hossein Mousavi, his defeated challenger, showed their defiance by shouting Allahu Akbar from the rooftops while the President was speaking. Some also engaged - unsuccessfully - in a new form of civil disobedience: turning on all their electrical appliances to try to overload the power grid when state television broadcasts the speeches by regime leaders or forced “confessions” of demonstrators.

Before the events of the past month, the student protests of July 1999 were the most serious challenge to the regime in the Islamic Republic’s 30-year history.

A peaceful demonstration at the University of Tehran against the closure of a reformist newspaper provoked a brutal attack by several hundred police and basiji. The unrest spread to other cities, and by the time it was suppressed a week later hundreds of students had been injured or locked up.

Mr Mousavi has neither endorsed nor discouraged tomorrow’s planned demonstrations, but increasingly he is seeking to challenge the regime legally and politically rather than through street protests.

This week he met Mohammed Khatami, the former president, and Mehdi Karoubi, another defeated presidential candidate, and the three men issued their first joint statement demanding an end to the security crackdown and the release of all detainees.

Hillary Clinton, the US Secretary of State, called for “even stricter sanctions” on Iran to try to “change the behavior of the regime”.

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