These are the first major student-led protests in months

According to reports, around 3,000 demonstrators gathered outside the university campus to support a student protest.

Riot police were called to the scene to try and calm the situation down.

It follows a similar protest on Wednesday when a small gathering of students quickly swelled into a demonstration of more than a thousand people.

On Wednesday, officials said they had released most of the 80 protesters they had detained in the first student-led protests in Iran in months.

But those arrests failed to deter students from gathering again to protest against plans to privatise Iran's colleges.

TV channels

According to reports, clashes erupted between rival groups, with both sides throwing stones at each other.

Soon they were joined on the street by thousands of other Iranians who had heard reports from US-based satellite TV channels.

Miranda Eeles, the BBC's correspondent in Teheran, said slogans were shouted against the clerical authorities and many of the protestors chanted "death to Khamenei", the supreme leader, and called for President Khatami to resign.

Others shouted slogans such as: "Tanks, artillery and guns no longer have any power."

The demonstrators are being cheered on from overseas Iran's Resalat newspaper Iranian press review

According to Eeles, people are frustrated by the slow pace of change and now feel betrayed by Mr Khatami and his lack of progress towards reform.

Clashes then erupted after Islamic militia groups arrived on motorbikes, wielding chains and batons.

Hundreds of riot police were called in to try and disperse the demonstrators into the surrounding streets.

According to the Associated Press, several people were seen being carried away with head injuries.

'Provocation'

After Tuesday's protests, the Iranian authorities had warned "illegal activities" would not be tolerated.

Intelligence Minister Ali Yunesi was quoted as saying that many protesters had been "provoked by extremists inside the country and elements outside the country to chant illegal slogans".

Correspondents say he was referring to calls broadcast by US-based satellite TV channels for people to gather at the university.

Students last held mass protests at the end of last year in anger at a death sentence passed against reformist lecturer Hashem Aghajari for alleged blasphemy.

They were later backed by 120 Iranian MPs who criticised the punishments meted out to the student leaders.

The authorities have called on students not to repeat the events of 1999, when clashes with police lasted for three days and left at least one student dead.

The 1999 crackdown marked the worst anti-government clashes since the Iranian revolution in 1979.