Tehran, Iran (CNN) -- Demonstrators shouting "Death to the dictator" clashed with police in Iran on Monday as students took to the streets to mark a key national anniversary, witnesses said.

At least two clashes occurred at Revolution Square, where police attacked demonstrators with batons and chased them onto side streets, witnesses said.

A large number of security forces ringed Tehran University, where the gates were shut and large crowds inside also chanted "Death to the dictator," the witnesses reported. Pro-government crowds also inside the university chanted slogans and waved the flag of the Islamic Republic, witnesses said.

The witnesses asked not to be identified out of concerns for security.

CNN could not independently verify the reports. The Iranian government did not allow members of the international media witness any possible protests this week.

The state-operated Press TV acknowledged the protests.

"A number of anti-government protesters attempted to hijack the occasion to hold rallies in Tehran. Their efforts were foiled by the presence of security forces which are deployed in several parts of the capital," an anchor said while the station showed images of pro-government demonstrations.

The demonstrations are being held on Student Day, an annual observance when Tehran extolls the virtues of the Islamic Revolution. The holiday commemorates three university students killed in 1953 by security forces of the Western-backed Mohammad Reza Pahlavi, the shah of Iran. He was toppled from power during the revolution two decades later.

The students this year are demonstrating against the disputed June 12 presidential election. President Mahmoud Ahmadinejad was declared the overwhelming winner in what protesters say was a rigged election. The election result was met with nationwide protests and the imprisonment of hundreds of demonstrators. Allegations of torture, rape and other abuses have since emerged.

Ongoing prosecutions of protesters have resulted in death sentences for some.

Since the election, other key anniversaries have met with protests against the current leadership.

Protesters and police clashed November 4, the anniversary of the 1979 siege of the U.S. Embassy in Iran.

In September, demonstrators took to the streets in protest on Quds Day, an annual event that is meant to show Iran's solidarity with Palestinians.

On Monday, police manned major intersections. Shopkeepers, fearing violence, shuttered storefronts.

As the protests got under way, Iranian security and paramilitary forces tear-gassed, beat and arrested students on university campuses, said a group called the International Campaign for Human Rights in Iran.

According to the human rights organization, protests have taken place at the following universities in Tehran: Amir Kabir, Tehran, Sharif, Elm va Sanaat, Honar, Tehran Markaz, Sureh, and Tehran Shomal. Protests also have taken place at universities in Isfahan, Kermanshah, Shiraz, Mashhad and Tabriz, and at Agricultural University of Karaj, the rights group said.

Security forces fired plastic bullets at students and protesters outside the Amir Kabir campus, said the International Campaign for Human Rights in Iran.

The organization's reports could not be independently verified.

In central Tehran, plumes of smoke rose from a large garbage bin that had been set on fire, witnesses told CNN.