The Iranian authorities say they have arrested a number of vigilantes supporting the hardline clerical leadership, following an attack against a student dormitory.
Students were attacked with knives
About 50 students are believed to have been wounded in Saturday's dawn raid at Tehran university - which followed four nights of violence triggered by student protests.
Students said dozens of armed Islamic vigilantes - who pledge allegiance to Ayatollah Khamenei - attacked students sleeping in their rooms.
On Saturday, Tehran radio said a number of "rioters" had been arrested following Friday night's disturbances, as well as people who had acted "arbitrarily".
The US has denounced action against the student-led protests and called on the Tehran government to protect demonstrators' rights.
Iran's supreme leader, Ayatollah Ali Khamenei, had warned the demonstrators they would be shown no pity if they did not stop.
Reports from Tehran late on Saturday said tension rose as night fell, with police forming a protective ring around the university.
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.
Fists and kicks
"Suddenly we heard windows being smashed," a student told the Associated Press news agency.
"Fists and kicks by hardline vigilantes woke up some of the students held up in their rooms."
Tehran quoted a statement by the hardline judiciary that those arrested included people who had attacked a dormitory.
"The majority of those detained are thugs and ruffians with criminal records," the statement said.
The radio said those arrested included Saeed Asghar, a vigilante leader who shot and wounded a top adviser to reformist President Mohammad Khatami in 2001.
The state-run news agency Irna said that in order to restore security, police would arrest anyone believed to be taking the law into their own hands.
BBC Iranian affairs analyst Sadeq Saba says conservatives in the Iranian leadership for many years have been using hardline Islamic groups as one of the most effective tools to suppress any challenge to their rule.
The attacks followed a fourth night of clashes on the streets of Tehran.
Security forces and hundreds of vigilantes used tear gas, clubs, chains and iron bars to disperse the protesters.
Students have staged protests for four nights
The BBC's Miranda Eeles in Tehran says militants from groups such as Bisji and Hezbollah were harsh in their methods - she saw one protester carried off on a motorbike wedged between two vigilantes, with a chain tight around his neck.
But despite the tough treatment, and the 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.
In an effort to thwart the crowds, police had completely sealed off the Tehran campus - the centre of the protests, which were sparked by plans to privatise the university.
However, nearby streets were jammed with cars sounding their horns, filled with ordinary Iranians who had turned out to express their support for the students.
The authorities have accused the United States and Iranian exiles of fomenting the unrest.
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