Middle East Correspondent Jim Muir: "They ended their letter with a warning"

In a letter to Mr Khatami, the military leaders say his reformist policies have led to chaos and they warn that their patience is running out.

The letter was written last week at the height of the unrest and has now been published in a conservative Tehran newspaper.

Twenty-four senior officers in the Revolutionary Guard signed the letter, including the commanders of Iran's land, sea and air forces.

Overall commander General Yahya Rahim Safavi was not a signatory but his views are known to be equally hard-line.

Correspondents say the attack marks an intensification of the power struggle between conservatives and reformists.

Student arrests

The Iranian Intelligence Ministry has said that a student it has arrested was behind the recent unrest in the country.

It said Manuchehr Mohammadi, accompanied by a colleague, Gholam Reza Mohajeri, had taken part in meetings abroad with Iranian counter-revolutionary groups and western human rights organisations during a recent four-month visit to the United States, Turkey and other European countries.

Mr Mohajeri was also detained. The authorites say both men are mercenaries in the pay of foreign powers and Iranian anti-government groups.

The ministry said both had received instructions from abroad to set up illegal student organisations with the aim of promoting the restoration of the Iranian monarchy and creating tension.

Correspondents say that Mr Mohammadi was active in a small nationalist students' union and had been briefly detained after his foreign trip.

Releases

Meanwhile, 750 people who were arrested during the unrest have been released, the Tehran Times newspaper reported on Monday.

It quoted an unnamed government official as saying 450 other people remain in detention, and that their cases are "under investigation".

The daily Arya also reported that five student leaders have been released. It was not clear whether they were among the 750 reported freed.

The protests began when police stormed a Tehran University dormitory on 9 July, hours after students rallied to protest the banning of a liberal newspaper, Salam.

In large-scale demonstrations which followed in subsequent days, three people were killed.

The Iranian authorities have blamed the unrest, the worst since Iran's 1979 Islamic Revolution, on the Iraq-based Mujahedeen Khalq, the largest Iranian opposition group.