A protest of around 1,000 supporters of defeated Iranian presidential election candidate Mir Hossain Mousavi has been broken up.

Bullets were fired and tear gas was reportedly used by riot police, backed by armed members of the feared Basij militia, at Haft-e Tir square in the centre of the city where hundreds had massed for the opposition rally.

The gathering came just hours after protesters were warned by the country's elite Revolutionary Guards, who are affiliated with the militia, that any election-related unrest would not be tolerated.

Mr Mousavi - beaten by President Mahmoud Ahmadinejad in the June 12 polls - has called for protests to continue. The country has witnessed the most violent unrest since the 1979 Islamic Revolution which ousted the Shah.

Authorities say the protesters are "terrorists", and following a warning during Friday's address by Iran's supreme leader, Ayatollah Ali Khamenei, Tehran's police commander Azizullah Rajabzadeh said officers will "confront all gatherings and unrest with all its strength".

Meanwhile, a senior Iranian politician has said reformist former prime minister Mr Mousavi faces legal action for acting against national security by calling for the street protests which have involved tens of thousands of people.

Ali Shahrokhi, head of parliament's judiciary committee, said: "Mousavi's calling for illegal protests and issuing provocative statements have been a source of recent unrests in Iran ... Such criminal acts should be confronted firmly. The ground is paved to legally chase Mousavi."

On Saturday, at least ten people were killed and more than 100 people injured in Tehran protests while gunfire was heard in the north of the capital - a bastion of support for Mr Mousavi - as helicopters hovered overhead.

A day later, Mr Mousavi wrote on his website: "Protesting against lies and fraud (in the election) is your right. In your protests, continue to show restraint. I am expecting armed forces to avoid irreversible damage."

Mr Mousavi said the mass arrest of his supporters "will create a rift between society and the country's armed forces".

US President Barack Obama, in the forefront of diplomatic efforts to halt an Iranian nuclear programme the West fears could yield atomic weapons, has urged Iran to stop violence against protesters.

Meanwhile, Britain is withdrawing the families of embassy staff working in Iran as a result of the violence, the Foreign Office has said.

A spokesman said: "The ongoing violence has had a significant impact on the families of our staff who have been unable to carry on their lives as normal.

"As a result, we are withdrawing dependants of embassy staff until the situation improves."

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