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 .
Authorities prepare for gatherings that could turn into demonstrations against elections . Students Day marks anniversary of 1953 killing of three students by shah 's security forces . Day is also a `` symbol of Iran 's struggle against tyranny , '' says ILNA news agency .