Tehran (AFP) - Iranian protesters defied police firing tear gas on Monday as they used an annual commemoration to stage fresh anti-government demonstrations despite a crackdown by security forces, witnesses said. Clashes were reported between protesters, mainly university students, and policemen at several universities and prominent districts of Tehran, which was flooded with security forces who arrested several demonstrators, they said. Among those reported held was student leader Majid Tavakoli from the prominent Amir Kabir university. Police fired tear gas at groups of protesters chanting slogans against President Mahmoud Ahmadinejad in Vali Asr intersection and Enghelab Street, one witness said. AFP could not independently confirm the incidents as foreign media were banned from covering Mondays annual Students Day protest. Protesters chanted Death to the Dictator and Do not be scared. We are all together, a witness said, adding that a policeman was also beaten up. Opposition website Mowjcamp.com reported that police also used tear gas against protesters in the popular Haft-e-Tir and Ferdowsi squares. Another witness described events as a cat and mouse game with protesters being chased by Basijis (Islamist militia) at several squares and bylanes in central Tehran. Websites reported anti-Ahmadinejad protests at Tehran University, Sharif University, the University of Fine Arts, Amir Kabir University and universities in the cities of Kermanshah and Mashhad. The official IRNA news agency said rioters tried to enter Tehran University. A number of rioters who wanted to misuse Students Day had gathered in streets adjacent to Tehran University. They clashed with police as they tried to enter the campus, it said. The elite Revolutionary Guards had warned of a crackdown on any attempt by regime opponents to hijack the annual Students Day, which marks the 1953 killing by the shahs security forces of three students. Students at Amir Kabir University ignored the warning and early on Monday urged people to protest. We are asking all people to come to universities so we can have one voice to protest at the coup detat, they said in an online statement posted under the name Green university students of Iranian universities. Green was the signature colour of main opposition leader Mir Hossein Mousavi in his campaign for the June 12 presidential election. He lost to Ahmadinejad in what he said was a fraudulent election staged to return the hardliner to power. Since then, Mousavi supporters have staged street protests at the slightest opportunity, accusing Ahmadinejad of stealing their votes. Another witness told AFP that around 1,000 policemen had surrounded Tehran University. Plain-clothes policemen were filming the events inside the campus and two protesters were also arrested, the witness said. Protesters were shouting slogans like Ya Hossein, Mir Hossein and Death to the Dictator, as some of them clashed with plain-clothes personnel inside the campus. Pictures obtained by AFP showed students covering their faces with green masks and scarves as they chanted anti-Ahmadinejad slogans inside the campus. Fars news agency said protesters broke windows of the universitys technical college, where the three students were killed in 1953, while protesters also broke a gate at Amir Kabir university. At Tehrans Sharif University, some students staged a symbolic funeral in honour of those killed in 1953. Neither Mousavi nor another opposition leader, reformist cleric Mehdi Karroubi, issued calls for protests on Monday, but both issued statements challenging the authorities. If you silence all the universities, what can you do with the situation of the society? Mousavi asked on his website Kaleme.com. Karroubi defended student protests and warned against a crackdown. The solution to arrive at reconciliation is tolerance and acceptance of criticism. We need to work to restore the trust between the authorities and the people, the cleric told the French daily Le Monde. Repression is not at all the solution, neither today nor tomorrow. Top cleric Ayatollah Naser Makarem Shirazi urged the warring political factions to unite. There was a majority and a minority in the (June) election and we have to create an understanding between them. I do not think this path is closed, ILNA news agency quoted him as saying.