Several thousand students openly defied a government ban on all rallies and official gatherings and took to the streets in the most serious unrest since the 1979 Islamic revolution.

President Khatami accused student leaders of "attacking the foundations of the regime and of wanting to foment tensions and disorder".

Hardliners who oppose President Khatami's social and political reforms plan to hold a rally on Wednesday in an apparent attempt to show that they, too, can muster up strong shows of support.

Baqr Moin, head of the BBC's Persian Service: Khatami's reforms will be helped if students stay within the law

State-run Tehran radio, which is controlled by the hardliners, repeatedly urged people to turn out in force.

The Iranian Defence Minister, Ali Shamkhani, told Iranian television that the events had begun with student demands that were "lawful and logical".

But, he went on, others were trying to take advantage of the student protests to further "their own interests and the interests of their masters".

Order would be restored on Wednesday "at all costs," he said.

Shots fired

James Robbins reports: "A week of clashes has grown steadily more serious"

During Tuesday's clashes, plain-clothed security forces are reported to have fired shots into the air to disperse students in central Tehran.

There were running street battles in several parts of the city.

Police fired tear gas and many of the students were arrested and taken away in police trucks after the crowd ignored an order from police circling in helicopters using megaphones to disperse.

"We don't want a government of force, we don't want a mercenary police," students chanted.

BBC Middle East Correspondent Jim Muir says a spiral of repression and grievances seems to have been set in motion, with unpredictable results.

Iranian State TV interrupted its normal programming to appeal for calm and warn of a plot to destabilise the country.

Thousands attended the funeral of a cleric in the north-west town of Tabriz, one of two deaths during the unrest which have so far been officially acknowledged.

Iranian radio said that a member of the opposition Mojahedin-e Khalq Organisation had been arrested over the shooting on Sunday.

Earlier Iran's conservatives moved to reassert their authority.

State radio and television, controlled by the conservative faction, repeatedly broadcast a speech by the country's spiritual leader, Ayatollah Ali Khamenei, who blamed the unrest on outside "enemies", mainly the United States.

On Monday, the governor of Tehran province banned all demonstrations in an attempt to halt the increasingly turbulent protests, declaring all demonstrations illegal.

The week of unrest was sparked by the closure of the liberal Salam newspaper last week.

Iran's press has enjoyed relative freedom since President Mohammad Khatami's election in 1997, but has come under mounting pressure from hardliners in recent months.

Spreading protest

Protests have also erupted in several other cities in Iran: the national news agency reported student demonstrations in Yazd in central Iran and in the western cities of Khorramabad and Hamadan, and in the north in Shahrud.

They are calling for the dismissal of Iran's hardline police chief, General Hedeyat Lotfian, and for control of law-enforcement agencies to be transferred from the Islamic authorities to President Khatami.

The government dismissed two security chiefs and reprimanded a third. But there has been no word of any move against General Lotfian.