Iran's hard-line judiciary acknowledged for the first time on Saturday that at least three prisoners detained after June's disputed presidential election were beaten to death by their jailers, confirming a key claim by the country's opposition movement.

Iranian anti-government student protests continued on Sunday as debate raged over television images showing the burning of a photo of the revered Ayatollah Ruhollah Khomeini, the founder of the Islamic republic.

Security forces and pro-government militiamen clashed with protesters shouting "death to the dictator" outside Tehran University on Monday, beating men and women with batons and firing tear gas, on a day of nationwide student demonstrations, witnesses said.

Government opponents shouted "Allahu Akbar" and "Death to the Dictator" from Tehran's rooftops in the pouring rain on the eve of student demonstrations planned for Monday. Authorities choked off Internet access and warned journalists working for foreign media to stick to their offices for the next three days.

Iran's supreme leader Ayatollah Ali Khamenei warned opposition groups they will be "confronted" if they jeopardise the nation's security, in an address during Friday prayers in Tehran.