Twenty police officers have gone on trial in Iran accused of taking part in an violent attack on a student dormitory last July.

The night-time raid provoked some of the worst riots in Iran since the 1979 Islamic revolution.

The main defendant is Teheran's former acting police chief, Brigadier Farhad Nazari.

He was dismissed after the raid, and strongly criticised in a government report into the attack, which was aimed at breaking up a student protest over the closure of a reformist newspaper.

Brig Nazari is charged with ordering the unauthorised entry of riot police into Tehran University grounds, disobeying orders and damaging the reputation of the security forces.

Alongside him, 19 other police officers face charges of assaulting and injuring students and destroying property. They face jail sentences of unspecified lengths, depending on the discretion of the judge.

The case marks a rare prosecution of members of the powerful security forces.

The BBC's Jim Muir at the military court in Tehran says the fact that proceedings are being held in public is a big step forward for reformist President Mohammad Khatami's new spirit of accountability and the rule law.

Hostel raid

At least, three people were killed in Tehran and the north-western city of Tabriz in the six days of unrest which followed the police action.

At least one person died and 20 were injured in the attack on the dormitory itself.

A succession of students testified in court on Tuesday that riot police, accompanied by right-wing vigilantes, stormed their sleeping quarters last July.

One, Raamin Karimi, said he was beaten, then thrown out of a third-floor window, before being beaten and kicked again, despite having broken his right leg and an arm in more than a dozen places.

'Rogue agents'

In addition to the criminal charges, court officials say some 400 students have filed complaints against the police for injuries and damage to personal property.

"Only some of the culprits have been pursued but others have been left alone," Mohsen Rohami, a pro-reform cleric acting as lawyer for the student plaintiffs, told the court.

Some 300 people, including relatives of students allegedly assaulted, were present at the trial.

Reformists, fresh from victory in recent parliamentary polls, have contrasted the slow progress of proceedings against the police with the speedy prosecution and conviction of student demonstrators.

Dozens of demonstrators have already been brought before the courts and death sentences have been confirmed on one of their alleged ringleaders.

Presiding Judge Akbar Tabatabai said the court would reconvene on Saturday to hear further testimonies.