Iranian police have moved in to break up crowds outside Tehran University during a protest by about 3,000 students calling for political reform.

The students were chanting ''freedom for political prisoners" and demanding a referendum on Iran's political future.

The police, who had built a barrier of buses to block views of the protest, clashed with a crowd outside the university gates and arrested about a dozen people.

Our national interests should be determined by the direct vote of the people

Student Mousavi Panah

The violence started with an exchange of stone-throwing between students and Islamic hardliners outside the university gates.

Several windows were broken in the barricade of buses and police moved in with truncheons to disperse the crowd.

The students had been authorised to mark National Students Day by holding a rally only inside the university grounds.

About 30 hardliners had earlier evaded identity checks to get inside the university grounds, sparking heated verbal exchanges.

Judicial review

The rally followed several protests last month sparked by the death sentence imposed on reformist academic Hashem Aghajari for questioning Iran's clerical rule.

They were the largest pro-reform demonstrations in Iran for more than three years and quickly brought a ban on such protests.

President Khatami faced calls to resign

"Our national interests should be determined by the direct vote of the people," said Mousavi Panah, a member of Tehran University's Islamic Students' Association.

On Saturday, the students shouted ''referendum, referendum'' and repeated calls for the resignation of the chief of the country's judiciary, whom they blame for the harsh sentence on Mr Aghajari.

Mr Aghajari's sentence is now under judicial review following an order from supreme leader Ayatollah Ali Khamenei.

Under Iran's complex political structure, moderate President Mohammad Khatami's efforts to improve democracy and social freedom have been largely blocked by hardliners who control key unelected institutions.

Mr Khatami, who normally attends the National Students Day ceremonies, stayed away this year.

The BBC Tehran correspondent, Jim Muir, said this had brought some calls for Mr Khatami to resign and that one of the main reformist newspapers used its front-page headline simply to ask him why he was not attending.