Iranian security forces clashed with thousands of opposition supporters on Wednesday after they tried to turn a state-organised rally on the anniversary of the US embassy siege into an anti-government protest.

Eyewitnesses said there were tens of arrests as police used batons, tear gas and pepper spray to disperse the crowd, but no deaths were reported and gun shots were not heard.

The elite Revolutionary Guards and the police had warned reformists that they would face brutal suppression if they exploited the 30th anniversary of the US embassy siege.

But Iran’s opposition – which accuses Mahmoud Ahmadi-Nejad of stealing the June election – ignored the threats, demonstrating its continuing determination to use major political and religious occasions to voice opposition to the government.

The Iranian regime organises annual demonstrations to mark the 1979 siege of the US embassy as “the den of espionage” . It considers the hostage-taking of US diplomats for 444 days as a “second revolution” after the Islamic revolution earlier the same year.

As in the past, thousands of intermediate and high school students and members of the Basij militia – the voluntary arm of the elite Revolutionary Guards – were bused in and chanted “Down with USA” and “Down with Israel” outside the former US embassy.

However, for the first time the streets leading to the venue were occupied by the opposition who chanted “The Russian embassy [is] the den of espionage”, in protest at Moscow’s military co-operation with Iran.

Iran’s supreme leader Ayatollah Ali Khamenei complained last week about the disappearance of anti-US and anti-Israel slogans in demonstrations.

Security was tight outside the Russian and British embassies, where both anti and pro-government demonstrators had threatened to stage protests.

Pro-reform demonstrators shouted “death to the dictator” and called for the establishment of the “national green government”, a reference to the “green movement” of the June presidential election.

“I’ll stay here until prisoners are released and the government of Ahmadi-Nejad is removed,” one angry woman in her 40s told the riot police who were trying to disperse the crowd.

Iran’s opposition leaders, including Mir-Hossein Moussavi who still claims the election was stolen from him, had urged their supporters to use the rally to reiterate their demands for democracy.

There were no immediate reports that Mr Moussavi was present, but Mehdi Karroubi, another presidential candidate who also insists the election was fraudulent, did join the rally before being forced by the police to leave.

“My concern is not the victory of Moussavi anymore,” said another woman, a 37-year-old teacher. “I don’t want the Islamic republic anymore.”

Students gathered behind Tehran University’s main gate but were prevented by riot police from going into the streets. Some carried placards and pictures of Mr Moussavi demanding that their votes be heard, while others shouted “death to the dictator”.

A student at Tabriz University in northwest Iran also confirmed that hundreds of students had gathered on the campus for hours and chanted slogans against the government. There were similar reports from universities in other parts of the country, including in Mashhad, in the northeast. Clashes were also reported to have taken place in the streets of Shiraz, in southern Iran, and police dispersed a crowd in the central city of Isfahan.

Meanwhile, US President Barack Obama released a statement calling on the Iranian government to choose whether “to focus on the past, or whether it will make the choices that will open the door to greater opportunity, prosperity and justice for its people”.

Ayatollah Khamenei on Tuesday rejected any engagement with the US and insisted the positive gesture of Mr Obama did not mean any real change in practice.

The authorities deny vote rigging and have portrayed the unrest as a foreign-backed bid to undermine the Islamic state.

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