One witness, Mehdi Arabshahi, said the campus protest lasted more than two hours as several students chanted slogans against President Mahmoud Ahmadinejad's hardline administration.
''Students chanted against policies by Ahmadinejad's administration, which is imposing pressures on the universities and detaining activists,'' Arabshahi said.
Another witness, Abbas Kazemi, said the protesters chanted anti-war slogans aimed at the United States and Israel.
He said students from other universities joined in the protest and broke one of the university's gates. But, Kazemi said, there were no clashes with police and no one was detained.
Tehran state-run radio in a news brief confirmed that students held a protest at Tehran University, saying the students chanted slogans against officials. It also said a group of non-students entered the university after breaking one of the gates but provided no other details. The media was not allowed to enter the university.
Female students protest for women's rights at demonstration (Photo: Reuters)
The university and its surrounding neighborhood were calm after the gathering and welders were repairing the broken gate.
The protests were held to mark National Day of Students, which has been celebrated since 1953 when three Iranian students were shot to death by police during a protest to a visit by the then US Vice President Richard Nixon to Tehran.
The protests also came as state TV announced Sunday that Iran's Intelligence Ministry had detained a group of activists it described as hecklers who planned to stage an illegal gathering at Tehran University.
Quoting a statement by the ministry, the TV report said the activists, who came from various cities, entered the university using fake identification cards before they were detained.
The report said intelligence officers confiscated concussion grenades, illegal books, and statements alcoholic beverages from the detainees.
It did not elaborate on number of detainees or say when the arrests took place.
But last week, a group of leftist students said 33 students and activists including four women were detained Tuesday after they staged a protest on the Tehran University campus.
Students were once the main power base of Iran's reform movement but have faced intense pressure in recent years from Ahmadinejad's hard-line government, making anti-government protests rare.
Since October, students from different universities have staged occasional protests over educational shortages, the firings of liberal teachers and dentition of activists.
About 100 students staged a rare protest in October against Ahmadinejad, calling him a ''dictator'' as he gave a speech at Tehran University marking the beginning of the academic year.
The president faced a similar outburst during a speech in December 2006 when students at Amir Kabir Technical University called him a dictator and burned his picture.
The Associated Press contributed to this report