TEHRAN - Iranian police clashed in Tehran Monday with opposition demonstrators seeking to renew their challenge to the government six months after a disputed presidential election, witnesses said.

Police fired teargas at supporters of opposition leader Mirhossein Mousavi when a state rally marking the killing of three students under the former Shah turned violent.

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

Authorities shut down the mobile phone network in central Tehran to stop opposition protesters from contacting each other, the reformist website Rah-e Sabz said. At least two women supporters of Mousavi were among those arrested, it said.

The June 12 presidential election, which secured President Mahmoud Ahmadinejad's re-election, sparked Iran's worst unrest since the Islamic revolution three decades ago and exposed deep divisions in the establishment. Authorities deny allegations of vote-rigging.

Security measures taken by the authorities on Student Day displayed their determination to uproot the opposition movement, which Mousavi said Sunday would continue despite pressure.

Witnesses told Reuters police fired teargas and clashed with protesters in various Tehran squares to disperse them, but there was no independent confirmation because foreign media were banned from leaving their offices.

Police fired teargas at demonstrators in Vali-ye Asr Square ... they are clashing with protesters, said a witness.

Journalists working for foreign media were told by officials not to leave their offices to cover stories from December 7 to 9.

Another witness form Tehran's central Haft-e Tir square said Security forces are beating demonstrators, men and women. Some of them are injured and bleeding.

The official IRNA news agency confirmed the clashes, calling protesters rioters.

Iran's Revolutionary Guards and their Basij militia allies had warned the opposition not to use the rally to revive protests against the clerical establishment that took place after the June vote.

Riot police surrounded Tehran University, where the main state rally was held, to try to prevent opposition protests.

Police have covered metal bars around Tehran University campus with white cloth to prevent passers-by from seeing inside the university, said a witness.

There are hundreds of riot police, (they are) everywhere around Tehran University and nearby streets, said another.

Internet connections were slow or completely down Monday as had been the case in the past few days.

Witnesses said there were protests at other cities.

NO NETWORK

The opposition, which mainly relies on websites or mobile phone SMS text messages to reach its supporters, called on people to gather on Student Day, following similar protests and clashes with police in September and November.

University students, who form a core of the opposition movement in Iran, urged people to join them. Reformist websites said anti-government protests were held inside at least three other universities.

The moderate Amirkabir website said police prevented students from leaving the university to join protests in other universities. Students are chanting Mirhossein we support you.

Another witness told Reuters dozens of plainclothes security forces had gathered in a northern Tehran square.

Mousavi encouraged his supporters Sunday to take to the streets to protest to the clerical establishment against the suppression of students, saying Ahmadinejad could not remain in power by ignoring people's votes, the Kaleme website reported.

The Mowjcamp website reported that dozens of students had been arrested in Tehran and other cities in recent days.

The Basij, alongside the Revolutionary Guards who are a key powerbase for Ahmadinejad, put down the June protests and arrested thousands of people.

Most have since been freed, but more than 80 people have so far been sentenced to up to 15 years jail and five people have been sentenced to death.

The reformist opposition says more than 70 people were killed in the post-election violence. Officials say the death toll was half that and included Basij militiamen.

(Writing by Parisa Hafezi; Editing by Andrew Dobbie)