"In total, 4,000 people were arrested across the country, and 40 percent of those arrested were immediately freed," Ayatollah Abdolnabi Namazi was quoted as saying Friday by the student news agency ISNA and semi-official news agency ILNA.

"Currently there are 2,000 people who are still in prison, among whom there are not many students," the ayatollah added, giving the first official figures for the number of arrests across the country.

In Tehran -- which was the epicenter of the June 10-20 demonstrations -- he said 800 people were arrested.

The protests began after a small student rally against the privatisation of some university facilities snowballed into anti-regime protests around Tehran university, sparking severe clashes between protestors and Islamist vigilantes.

The protests -- the most serious since the July 9, 1999 clashes in Tehran -- also spread across the country and were marked by the unprecedented shouting of virulent slogans against Iran's supreme leader Ayatollah Ali Khamenei.

The protests fizzled out after a tough crackdown, and after scores of people were seriously injured and hundreds detained. One demonstrator died in the southern city of Shiraz in circumstances that remain unclear.

Namazi was also quoted as confirming that Iran's authorities have banned students from commemorating next month the 1999 unrest, which saw at least one student die in riots that followed a heavy-handed police raid on a Tehran university dormitory. He said no events would be permitted on or off campus.

The tough crackdown has sparked widespread anger among students, and has placed yet more pressure on embattled reformist President Mohammad Khatami -- who has defended the right to protest but appears to wield little or no influence over the security forces, judiciary or intelligence service.

In an open letter received Friday, a group of 106 prominent student activists called on Khatami to step in to defend the right to protest or else resign.

"We ask you to prevent an uproar before it is too late by finding a good way forward. Otherwise, you must act bravely by resigning from your post so as not to legitimise the policy of repression," Khatami was told.

Also Friday, a prominent cleric and politician said in a sermon that the unrest was part of an alleged United States plot to topple the nearly 25-year-old regime, and hence those detained in the unrest should be shown little mercy.

"This chaos was predictable, as after all the Americans have been saying that they support the overthrow of the regime," Ayatollah Ahmad Jannati, a member of the Guardians Council legislative oversight body, told worshippers at Friday prayers in Tehran.

"Those who were arrested should be treated according to the law. There were a number of them who were fooled, but even they should also pay the price. This is no joke," he added, describing the protestors as "mercenaries, thieves, smugglers, hooligans and Savaki" -- the ousted shah's notorious secret police.

He also took aim at the US President George W. Bush and British Prime Minister Tony Blair.

"The United States should understand that this regime is not a regime to be toppled by chaos. Wicked Bush and Blair should understand that here, they are facing a regime whose people are ready to sacrifice their lives," he warned.

Meanwhile, Tehran's hardline chief prosecutor Saeed Mortazavi told ISNA that an undisclosed number of newspapers were also being investigated for their alleged role in the mid-June unrest.

"Certain newspapers that played a negative role by causing problems are under investigation," said Mortazavi, who is best known for his closing down of scores of pro-reform newspapers in his previous job as the head of Tehran's press court.

He also alleged that "certain deputies" in the Iranian parliament also played a role in the unrest, but did not say if they too were facing legal action.