A ministry spokesman said the arrest of Kazem Shokri, a senior editor with the Sobh-e-Emrouz newspaper, breached press laws.

The ministry said only the publisher of a newspaper could be held responsible for any offences under current press laws.

Mr Shokri has been charged with authorising the publication of an article alleged to be insulting to the Muslim holy book, the Koran.

The official Iranian news agency, IRNA, reported Mr Shokri was imprisoned on Tuesday 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.

Student warning

Student leaders in Iran have warned that future generations will turn to non-peaceful means to achieve reform if hardline conservatives continue to block the programmes of President Mohammad Khatami.

The warning came as 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.

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.

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.

Guards' letter

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

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