Thousands of pro-reform demonstrators have taken to the streets of the Iranian capital Tehran for the fifth successive night.
Students say they will continue the demonstrations
But most stayed inside their cars, fearing a repeat of the attacks by right-wing vigilantes on the previous demonstrations.
Reports say there was an overwhelming police presence, resulting in muted protests which passed off largely without incident.
The protests, which are now reported to have spread to other cities, have been against the conservative clerical establishment and the reformist President, Mohamed Khatami, who is accused of betraying hopes for change.
Riot police formed a ring around Tehran university to protect students from conservative vigilantes who stormed dormitories there the night before.
Police had earlier warned the vigilantes, who are loyal to the country's supreme religious leader Ayatollah Ali Khamenei, against taking the law into their own hands.
They have also issued a warrant for the arrest of Sa'id Askari, a prominent pro-government vigilante leader who was seen at the demonstrations.
And state radio reports police have arrested scores of what it describes as "thugs and ruffians" in the previous four nights of civil unrest in Tehran.
Despite this, the latest protests saw vigilantes gather close to the university, while others patrolled traffic jams and manned roadblocks.
The early hours of Saturday morning saw security forces and hundreds of vigilantes used tear gas, clubs, chains and iron bars to disperse protesters.
Some students were attacked with knives
Students said dozens of armed Islamic vigilantes were involved in the attack on the university dormitory.
The US has denounced action against the student-led protests and called on the Iranian Government to protect demonstrators' rights.
A White House statement reiterated support for democratic reform in Iran, demanding that the voice of the Iranian people be heard.
The Iranian authorities have accused the US and Iranian exiles of fomenting the current unrest.
The BBC's Miranda Eeles, in Tehran, says thousands of ordinary Iranians have joined the students in demonstrations.
Despite the rough treatment of protesters, and warnings from Ayatollah Khamenei, our correspondent says the demonstrators' resolve to continue speaking out against the slow process of reform shows no sign of waning.
Student associations have said they will continue to demonstrate until 9 July, to commemorate the violent attack by hardline groups on students four years ago.
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