Iranian police clashed in Tehran on 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.

At least two women supporters of Mousavi were arrested and 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.

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 on 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 have been banned from leaving their offices.

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

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

Iranian riot police have surrounded Tehran University, where the main state rally was held, to prevent the opposition from staging new demonstrations against the government.

"There are hundreds of riot police, (they are) everywhere around Tehran University and nearby streets," said another witness, who asked not to be named.

Internet connections were slow or completely down on Monday as has been the case over the past few days.

"No network"

The opposition, which mainly relies on its websites or mobile phone SMS text messages to reach its supporters, have called on people to gather on Student Day near the Tehran University, where the main state-rally will be held.

In September and November opposition demonstrators clashed with government backers and police at annual rallies.

University students, who form a core of the reform movement in Iran, have urged people to join them.

"Six universities in Tehran ... will host the people from outside universities to join the students at 1130 GMT," a reformist website reported.

On Saturday, Iran banned foreign media from reporting on the Student Day rally. Journalists working for foreign media were not allowed to leave their offices to cover stories from Dec. 7 to Dec. 9.

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

"Tens of plainclothes with motorbikes are at Tajrish square.

More of them are joining the group," said the witness.

Mousavi encouraged his supporters on 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.

Reformist website Mowjcamp said dozens of students had been arrested in Tehran and other cities in the past days.

The volunteer Basij militia, alongside the Revolutionary Guards which is a key powerbase for Ahmadinejad, put down the June protests and arrested thousands.

Most have since been freed, but more than 80 people have so far been sentenced to jail terms of up to 15 years and five people have been sentenced to death. The reformist opposition says more than 70 people were killed in post-election violence.

Officials say the death toll was half that and included Basij militiamen. – Reuters