The editor, Kazem Shokri, of the daily Emrooz was charged on Tuesday with having authorised the publication of an article offensive to the Koran.

The official Iranian news agency, IRNA, reported Mr Shokri was imprisoned after being summoned to court by the Tehran Justice Department over the article, "Two parallel lines do not cross unless God wills it".

Iranian hardliners have recently stepped up a campaign against liberal newspapers which have flourished under the reformist President, Mohammed Khatami.

Two weeks ago, the moderate Salam newspaper was closed, triggering six days of unrest during which at least three people died.

Earlier on Tuesday, student leaders in Iran warned future generations would turn to non-peaceful means to achieve reform if hardline conservatives continue to block the programmes of President Mohammad Khatami.

The warning came on the day that student representatives met officials from the Supreme National Security Council to press their demands for change.

Leaders of the most prominent student organisation, the Unity Consolidation Office, contradicted the official version of the reasons behind the unrest on the streets of Tehran last week.

Much of the official account has dwelt heavily on the role of outside countries and counter-revolutionary groups based abroad in allegedly stirring up the trouble.

But one of the student leaders, Ali Afshari, said the real cause of the violence lay closer to home - in the frustration felt by millions of young people at the slow pace of change.

Foiled

Iranian Affairs Reporter Sadeq Saba: "The students do not seem hopeful of getting reforms by peaceful means"

They voted for the reformist president, Mohammad Khatami, two years ago, only to see his attempts to bring about change constantly foiled by hardline conservatives.

The student leaders warned that Mr Khatami and his reform programme represented the last chance for the Islamic regime.

If it failed, they said, future generations of students and young people would turn to non-peaceful means to achieve their goals.

Mr Afshari also challenged the official account by asserting that the street riots were the work of hardline provocateurs bent on undermining Mr Khatami's position.

Warning letter

The riots prompted a letter to the president from 24 senior commanders of the Revolutionary Guards, blaming him and warning that their patience was running out.

The letter did not say what action would be taken, but the warning lends an ominous tone to the pressure President Khatami faces in his struggle with Islamic hard-liners.

The letter said the president's moves toward greater democracy were leading the Islamic republic into "anarchy".

Mr Afshari said the letter had aggravated worries that the ground might be being prepared for a coup d'etat against the president.

President Khatami has played down the importance of the letter, which was published by conservative newspapers on Tuesday.

The president's office said such correspondence was perfectly normal, but what was worrying was the publication of such highly confidential material in the press.