The plainclothes Basijis beat protesters on the heads and shoulders as the crowd scattered, then regrouped on nearby street corners. Nearby, protesters and Basijis pelted each other with stones, the witnesses said, speaking on condition of anonymity for fear of retaliation.
Pro-reform campus groups have called for students across the country to turn out Monday for massive rallies at universities against Iran's clerical leadership -- the first major protest in more than a month. The opposition has been struggling to maintain its presence on the streets after security forces crushed massive rallies that erupted over Iran's disputed presidential election in June.
While turmoil erupted in the streets outside Tehran University on Monday, authorities took dramatic steps to close the campuses to the outside world.
Cell phone networks around the universities were shut down. To hide anything going on inside, the fence around Tehran University was covered with banners and signs bearing quotes from Supreme Leader Ayatollah Ali Khamenei and messages marking an important Shiite occasion celebrated Sunday. Police and members of the elite Revolutionary Guard surrounded all the university entrances and were checking IDs of anyone entering to prevent opposition activists from joining the students, witnesses said.
Footage posted on YouTube purported to show a march by thousands on Monday inside Tehran University. The young men and women were shown chanting "death to the dictator" and slogans against the Basij as they marched through the campus. There was no sign of security forces, and the students marched without frictions. The authenticity of the footage could be immediately be confirmed.
At Tehran's Amir Kabir University, Basiji militiamen entered the campus and tried to break up a march by several hundred students, witnesses said. The Basijis pushed and shoved the students, dragging some away.
The potential for violence inside the campuses was high. Some 2,000 Basiji students were brought into Tehran University early Monday -- obstensively to hold a celebration for the Shiite holiday, but such hard-line students are often used to crush pro-reform rallies on campus.
Journalists working for foreign media organizations are banned from covering Monday's planned protests. They were told late Saturday by the Culture Ministry that their press cards would be suspended for three days starting Monday.
Government opponents were hoping for a large turnout for Monday's demonstrations to show their movement still has momentum despite a series of government crackdowns since the election, which the opposition says President Mahmoud Ahmadinejad won by fraud.
Opposition leader Mir Hossein Mousavi threw his support behind the planned student demonstrations and declared that his movement was is still alive. A statement posted on his Web site said the clerical establishment cannot silence students and was losing legitimacy in the Iranian people's minds.
"A great nation would not stay silent when some confiscate its vote," said Mousavi, who claims to be the real winner of the June 12 vote.
Khamenei, the supreme leader who has final say on all state matters, accused the opposition Sunday of causing divisions in the country and creating opportunities for Iran's enemies.
Students at Tehran University played a major role in street demonstrations in support of the 1979 Islamic Revolution that toppled to pro-U.S. shah and brought clerics to power. But in the past decade, universities have become strongholds for the pro-reform opposition, which seeks to reduce the clerics' domination of politics.
Sunday night, rooftop cries of "Allahu akbar" or "God is great" and "death to the dictator" were heard from many parts of Tehran in support of the opposition. The rooftop chants -- which were almost every night in the weeks following the election -- had not been heard since the opposition's last attempt to mobilize, a Nov. 4 rally coinciding with state-sanctioned events to mark the anniversary of the 1979 U.S. Embassy takeover.
That demonstration drew far fewer protesters than at the height of the summer's unrest. But it still provoked a violent response from security forces. Monday's protests appears to be larger than the Nov. 4 rallies.
For weeks after the disputed June presidential election, demonstrations triggered by claims of massive fraud in the vote brought hundreds of thousands to the streets, but the relentless crackdown that followed has taken a heavy toll.
Seeking to deny the protesters a chance to reassert their voice, authorities slowed Internet connections to a crawl in the capital. For some periods on Sunday, Web access was completely shut down -- a tactic that was also used before last month's demonstration.
The call for Monday's demonstrations was put out on dozens of Web sites run by supporters of opposition leaders Mousavi and Mahdi Karroubi, who both ran against Ahmadinejad in the June 12 election. Most of those sites have been repeatedly blocked by the government, forcing activists to set up new ones.