Witnesses say hundreds of police used batons and tear gas to disperse demonstrators gathered in the streets of central Tehran.
Thousands of Iranian students have turned out for protests against the government. Iranian security forces tried to block access to the rallies, but clashes with noisy protesters erupted at several sites.
A large crowd of students at Tehran University chanted slogans against President Mahmoud Ahmedinejad at an angry protest rally meant to symbolize student opposition to the government. December 7, known as "Student Day," commemorates the 1953 slaying of three students during a protest under the late Shah.
Security forces fired tear gas in an effort to block some of the rallies. Pro-government Basij militiamen also clashed with students in many places, hurling bottles, chunks of wood, and stones at them.
Foreign news agencies were forbidden from covering the much-anticipated student rallies, and government security forces deployed on and around campuses, nationwide, blocking entrances and pushing back crowds.
Opposition leader Mir Hossein Mousavi warned the government it is "fighting the shadows in the streets," by trying to suppress the student rallies, and told those in power that they would be powerless to "quash" the protests.
Mr. Mousavi claims to have won a disputed June 12th presidential election, amid charges of vote-rigging by the government.
A video on Mir Hossein Mousavi's Facebook website showed scores of student demonstrators near the main gate of Tehran's Sharif University, shouting "traitor get lost" in a disparaging reference to incumbent President Mahmoud Ahmedinejad.
An Iranian émigré radio station in Los Angeles also talked with demonstrators, capturing sound of a crowd denouncing pro-government Basij militiamen on a Tehran campus. Hundreds of Basij militiamen were reported to have been deployed by the government on and near campuses in most Iranian cities.
Demonstrations by students in other Iranian cities were also reported to be widespread. Hundreds of student protesters can be heard chanting slogans against the Iranian government, as they mulled in front of a campus administration building at Mashhad's Azad University, according to a video also posted on Mr. Mousavi's website.
Photos posted on Twitter and opposition websites showed students burning pictures and posters of both President Ahmedinejad and Supreme Leader Ayatollah Ali Khamenei at an unspecified campus.
The government appears to have curtailed mobile phone service in many areas of Tehran, and press reports say internet service has been severely disrupted as well.
Iran's elite Revolutionary Guards, as well as top police officials, had warned that they would deal harshly with attempts to demonstrate against the government on the anniversary.