Dozens of militants stormed at least two university dormitories, beating up students in their beds and detaining several of them as violence aimed at silencing government critics raged through Iran's capital.

Across the city, supporters of Iran's hard-line clergy beat pedestrians with clubs, brandished knives, fired machine guns in the air and hurled rocks at homes earlier Friday night. It was the most intense and widespread fighting in four consecutive nights of violence in Tehran.

The attacks were aimed at intimidating young protesters who have gathered in Tehran (search) for nightly demonstrations this week calling for an end to the country's hard-line establishment led by supreme leader Ayatollah Ali Khamenei (search).

Student Mojtaba Najafi said about 200 students were sleeping in their rooms in the Hemmat dormitory of Allameh Tabatabai University (search), when the attacks began. He said over 50 students were injured and taken to the hospital and about two dozen had disappeared after the attack.

"We were sleeping in our beds. Suddenly we heard windows being smashed," Najafi said. "It was the most brutal way of attacking a human being. They beat up the guard before entering our dormitory. They see no borders, no limits."

There were also attacks at Tehran University's Chamran dormitory before dawn Saturday, other students said.

In 1999, a similar attack on a dormitory led to the death of at least one student and generated the worst street battles since the 1979 Islamic Revolution.

Khamenei referred to the 1999 raid this week when he warned of a major crackdown in a speech broadcast on state television and radio.

Unlike previous nights, no anti-clergy demonstrations were seen before the violence erupted Friday night.

Protesters had been calling for an end to the country's hard-line establishment and for supreme leader Khamenei's death. Criticism of Khamenei is usually punished by imprisonment, and public calls for his death had been unheard of until this week.

Khamenei's hard-line supporters — who control key government institutions in Iran — are locked in a power struggle with popularly elected President Mohammad Khatami (search), who came to power aiming to reform Iran's conservative political system.

Although the protesters criticize the clerics, public support for Khatami also appears to be falling because of his government's inability to implement promised reforms.

During a Friday prayer sermon at Tehran University, former president Hashemi Rafsanjani (search) — a key Khamenei supporter — urged Iran's youth not to fall into what he described as a U.S. trap by denouncing the country's political leadership.

"I advise the youth, especially students ... that they should be careful not to fall into trap dug out by the Americans," Rafsanjani told worshippers.

Rafsanjani said it was Iran's policy to "act resolutely" to prevent demonstrations from getting out of hand but "not to be harsh on protesters."

While the recent protests seemed to be disorganized, with no clear leaders, some demonstrators vowed to keep up the pressure until next month's anniversary of the much larger and violent protests in 1999.

Exiled opposition groups have been encouraging dissent in Iran through U.S.-based Persian language TV channels. U.S. pressure on Iran, which Washington accuses of hiding a nuclear weapons program and harboring terrorists, may have further emboldened those who hope to see the regime toppled.