Security forces and militiamen clashed with thousands of protesters shouting "death to the dictator" outside Tehran University on Monday, beating them with batons and firing tear gas on a day of nationwide student demonstrations, witnesses said.

The protests were the largest in months, as university students — a bedrock of support for the pro-reform movement — sought to energize the opposition with rallies at campuses across the country. The opposition has been reeling under a fierce crackdown since turmoil erupted over the disputed presidential election in June.

Thousands of riot police, Revolutionary Guard forces and pro-government Basij militiamen flooded the area around Tehran University since the morning, vowing to prevent any unrest from spilling out into the streets.

Banners and signs bearing slogans from Supreme Leader Ayatollah Ali Khamenei blanketed the tall campus fence, hiding whatever took place inside. Cell phone networks around the universities were shut down, and 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.

Slideshow: Iranian Forces, Protesters Clash

The heavy clampdown raised fears of an escalation of violence during Monday's clashes.

"There's anxiety that there will be violence and shooting. I shout slogans and demonstrate but try not to provoke any clash with the security," one Tehran University student, Kouhyar Goudarzi, told The Associated Press in Beirut by telephone. "We are worried."

Clashes erupted when thousands of protesters massed in the streets outside Tehran in support of the students. As they chanted "death to the dictator," riot police fired tear gas and Basij militiamen charged the crowds, the witnesses said.

The plainclothes Basijis beat protesters on the head 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.

Inside the university, thousands of students marched through the campus, many of them wearing surgical masks or scarves over their faces to protect against tear gas. Some wore green wristbands and waved green balloons, the color of the opposition movement of Mir Hossein Mousavi.

Footage posted on YouTube purported to show thousands protesting inside Tehran University, chanting "death to the dictator" and slogans against the Basij — but no sign of riot police security forces. The authenticity of the footage could not be immediately be confirmed.

Some protesters scuffled with hard-line students who were holding a counter-protest on the campus. The two sides pushed and shoved in a crowd, according to witnesses. The hard-liners — numbering about 2,000 — marched through the university, waving pictures of Khamenei and Iranian flags and chanting "death to the hypocrites," a reference to Mousavi and other opposition leaders.

At Amir Kabir — another of several universities around the capital — 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.

But it appeared that regular riot police and other security forces were largely staying out of any protests inside the campuses, a sign they aimed to bottle them up while focusing on unrest in the streets.

Authorities have arrested well over 100 student leaders in past weeks, looking to blunt Monday's protests. On Saturday, police detained 15 women from the Committee of Mourning Mothers, which groups relatives of protesters who have been killed in Iran's postelection crackdown. The women were arrested at a Tehran park where they have held weekly protests for months, the International Campaign for Human Rights in Iran.

Journalists working for foreign media organizations, including the AP, were banned from covering Monday's protests. They were told late Saturday by the Culture Ministry that their press cards would be suspended for three days starting Monday.

Authorities also 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.

Opposition leader Mousavi threw his support behind the marches, declaring that his movement was still alive and that the clerical establishment 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 presidential election.

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.

The opposition says President Mahmoud Ahmadinejad won the election through fraud. For weeks after the election, hundreds of thousands marched in the streets of Tehran against Ahmadinejad.

But the relentless crackdown that followed has taken a heavy toll. Protests in recent months have been far smaller, and the opposition has given up trying to hold consistent street rallies. Instead, it times demonstrations to coincide with significant national events to help drum up a crowd. Monday's protests were held on National Students Day, an annual occasion when student rallies are traditionally held.

Monday's protests appeared larger than the last major street demonstrations, on Nov. 4.

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.

The night before the protests, 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 November protest.

New York Times: Iran Arrests Moms Mourning Children.