Thousands of Iranians took to the streets of the capital, Tehran, in the first student-led demonstration for months. Residents say a protest over plans to privatize universities quickly developed into a political tirade against Iran's governing Islamic clerics.
Crowds shouting for political prisoners to be freed clogged the square near Tehran University, the scene nearly four years ago of the biggest pro-reform unrest since the 1979 Islamic revolution.
This time, though, the protesters railed against both Iran's powerful Muslim clerics for limiting their freedoms and the reformist government for failing to rein them in.
Demonstrators smashed shop windows, set fire to motorcycles and damaged public telephones before baton-wielding riot police and members of the hard-line Islamist Basij volunteer militia broke up the throng.
Student leaders say Tehran's security chief, Ali Taala, urged them to avoid a repeat of the violent rioting that gripped the Iranian capital in July 1999.
In a reference to mounting U.S. pressure on Iran, he said they should understand the situation the country finds itself in and ensure their activities do not get out of hand.
But analysts predict more unrest will follow. The standoff between President Mohamed Khatami and the reform-led parliament and powerful hard-liners in the judiciary has widened. Prominent reform activist Hashem Aghajari remains jailed and his sentence is still under review.
Analysts say there is enormous political and social pressure on Iranian society.
Earlier this week, Secretary of State Colin Powell urged Iranians to force change from within to make Iran what he called a less troublesome member of the world community. In recent week, U.S. officials have repeatedly accused Iran of trying to develop weapons of mass destruction and of harboring al-Qaida terrorists, charges that Iran denies.
Iran's supreme leader, Ayatollah Ali Khamenei, has urged the students to foil what he called a devilish American plot to destabilize the Islamic regime.