Thousands of Iranian police today used batons and teargas to disperse demonstrators chanting anti-government slogans in the latest street confrontation between the government and the opposition.

In the largest protests in months, university students held rallies at campuses across the country to show that the reform movement was still alive despite a fierce crackdown on dissent after the disputed presidential election in June.

The authorities shut down the mobile phone network in the centre of Tehran to stop opposition protesters from contacting each other, the reformist website Rah-e Sabz said. At least two women supporters of the opposition leader Mir Hossein Mousavi were among those arrested, the site said.

Clashes erupted as thousands of protesters massed in the streets outside Tehran in support of the students. As the crowd chanted "Death to the dictator," riot police fired teargas and Basij militiamen charged the crowds, according to witnesses.

The plainclothes Basijis beat protesters on the head and shoulders as the crowd scattered, then regrouped on nearby street corners. Protesters and Basijis pelted each other with stones.

"I saw at least 10 people being arrested and taken to minibuses," one witness said.

Footage posted on YouTube purported to show thousands protesting inside Tehran University, chanting "Death to the dictator" and slogans against the Basij, but there was no sign of riot police. The authenticity of the footage could not immediately be confirmed.

The clashes came as Iran commemorated the killing of three students in 1953 under the former shah. Security forces put on a show of strength to prevent the opposition from taking over the main state rally. The large security operation showed the authorities planned to make good on their promise to deal harshly with protesters. In recent years, students have used the occasion to hold pro-reform demonstrations.

"Police are using batons to disperse demonstrators. People are chanting anti-government slogans in the Ferdowsi Square," a witness told Reuters.

Other witnesses said police were conducting ID checks on anyone entering the Tehran University campus, to block opposition activists from joining the students. Security forces also sought to conceal the campus from public view, covering the main gate and the fence with banners carrying quotations by the supreme leader, Ayatollah Ali Khamenei, and congratulatory messages marking an important Shia occasion celebrated yesterday.

"There are hundreds of riot police, everywhere around Tehran University and nearby streets," a witness said.

Many shops and businesses outside the university were closed but the rest of Tehran appeared to be functioning normally.

Foreign media were banned from covering the protests. The culture ministry told them press cards would be suspended for three days starting today.

Last night, government opponents climbed on to rooftops and shouted "Allahu Akbar" and "Death to the dictator".

Internet access was restricted to deny the opposition a vital means of mobilising supporters.

The authorities have arrested more than 100 student leaders in past weeks, looking to blunt today's protests. On Saturday, police detained 15 women from the Committee of Mourning Mothers, consisting of relatives of protesters who have been killed in the post-election crackdown. The women were arrested at a Tehran park where they have held weekly protests for months, the International Campaign for Human Rights in Iran said.

. Mousavi threw his support behind the planned student demonstrations and declared that his movement was still alive. A statement posted on his website said that the clerical establishment could not silence students and was losing legitimacy in the Iranian people's minds.

"A great nation would not stay silent when some confiscate its vote," said Mousavi, who claims President Mahmoud Ahmadinejad stole the 12 June election victory from him.

Khamenei, who has the final say on all state matters, accused the opposition yesterday of exposing divisions in the country and creating opportunities for Iran's enemies.