The Iranian regime warned that any demonstrations would be mercilessly crushed, and meant it. As darkness fell on baking, dust-shrouded Tehran last night an army of riot police and hardline basiji militiamen used batons, gun butts and tear gas to beat back thousands of Iranians converging on the city centre.

"The security presence was massive. It was like a military occupation," one witness told The Times. "They were clubbing the hell out of people."

The greater victory belonged to the demonstrators, however. Male and female, some quite old, they came armed with nothing more than a burning sense of injustice. They defied the risk of serious physical injury, and the very real possibility of arrest, incarceration and torture. They did this to show the world that their resistance to Iran's brutal and illegitimate government has not been extinguished.

"We went today to show them that we are still here and are not going away and they can’t talk or scare us away. And we'll be back every time there is an occasion to commemorate or when we're asked to," said Maryam, a young female office worker nursing an arm injured by a baton blow. “We want to be heard. We are not going to let the regime ignore us,” said Ahmad, a young man in his twenties.

The demonstrations were the first since the massive street protests that followed Iran's hotly-dispute presidential election were finally suppressed nearly two weeks ago.

They were called to mark the tenth anniversary of the student uprising which erupted in 1999 after hundreds of basiji stormed the University of Tehran following a demonstration by reformists. That uprising was the most serious challenge the regime had faced since the Islamic Republic was founded in 1979, but it is dwarfed by the turmoil that has engulfed Iran since President Mahmoud Ahmadinejad defeated Mir Hossein Mousavi in what the opposition insists was a rigged election.

The regime did its best to prevent fresh manifestions of public anger today. It took 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. It shut down the text messaging system, and Morteza Tamadon, Tehran's governor, warned that demonstrations inspired by "anti-revolutionary networks" would be "trampled under the feet" of the security forces.

The demonstrators came anyway — not in the massive numbers of the earlier protests, and not with the banners or camera phones that would make them instant targets, but with even greater courage.

They were cowed neither by the regime's brutality, nor by security agents filming them so they could be indentified later. They held their hands aloft in victory signs. They chanted "Death to the dictator" and "Ahmadi be ashamed and let go our the country" and "Don't be afraid, we're all together". From all directions they sought to converge on Enghelab Square and the University of Tehran, but eyewitnesses said security forces on foot or motorbikes charged any group of more than a few hundred. The demonstrators would retreat, regroup elsewhere, and be attacked again.

Foreign journalists have been banned from Iran, but witnesses said that clashes continued after dark. Rubbish skips were set ablaze and the centre of Tehran reeked of tear gas. Police fired guns into the air. Basiji seized the number plates of cars that sounded their horns to show support for the opposition, or hit the vehicles with batons, but they could not silence the protest with physical force alone.

"The demonstrators made a moral point. They told the government in no uncertain terms that they're still there and not going away," said an Iranian analyst who witnessed the scenes of mayhem.

The millions of Iranians who no longer dare to demonstrate have not gone away either. They are channelling their anger into a campaign of civil disobedience. Apart from shouting "God is great" from their rooftops each night, they have started writing Mr Mousavi's name on banknotes, boycotting government banks and goods advertised on state television, and turning on all their electrical appliances at the same time to try to overload the electrical grid.

Mr Mousavi is seeking to form a political movement to challenge the regime. Earlier this week the former president Mohammed Khatami, and Mehdi Karoubi, another defeated presidential candidate, issued their first joint statement demanding an end to the security crackdown and the release of all detainees.

Amnesty International condemned Wednesday's violence. “Once again, the authorities have demonstrated their intolerance of dissent in a manner all too reminiscent of the ruthless methods they used in 1999," it said. “It is high time that they stop using strong-arm tactics to crush protest and abide by their obligations under human rights law."

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