Egyptian security forces have fired teargas in Cairo's Tahrir Square to disperse protesters demonstrating against the country's army-backed government.

Sunday's demonstration was the first by supporters of deposed president Mohamed Morsi in the iconic square - the epicentre of the 2011 revolt against former president Hosni Mubarak - since he was toppled by the army on July 3.

Several hundred people chanted "down, down with military rule" and one protester hung a picture of Morsi from a lamppost before the teargas volleys began.

The square has been the near exclusive domain of liberal and secular protesters since shortly after Morsi took office in June 2012.

The crowd on Sunday had come from Cairo University, where they were protesting the death of an engineering student at the hands of police.

The latest demonstration came as the country's constitution-drafting panel voted to retain military trials for civilians in certain cases despite opposition from some secular activists and rights groups concerned over the army's wide-ranging powers.

If approved, the constitution would be submitted to a popular referendum early next year, billed as the first stage in a "democratic transition" promised by the military-installed authorities.

The panel has now approved all but four articles, which deal with election procedures. It adjourned on Sunday for a private meeting to discuss the disputed articles and was expected to vote on them again later the same day.

Also on Sunday, authorities extended the detention of prominent secular activist Alaa Abdel Fattah by 15 days after he was arrested for holding an unauthorised demonstration against the provision in the draft charter allowing military trials of civilians.

His detention is expected to further anger secular activists who are furious over the provisions in the draft charter concerning the military.

Another 24 activists also saw their detention extended by 15 days on Sunday.

Military privileges

The issue of the military's longstanding privileges was at the heart of voting on the constitution on Sunday after the 50-member panel approved 138 of the 247 articles of the draft charter the day before.

The panel approved Article 204, which says that "no civilian can be tried by military judges, except for crimes of direct attacks on armed forces, military installations and military personnel."

Secular activists had demonstrated against the provision, fearing it could be applied to protesters, journalists and dissidents.

Such fears deepened after Abdel Fattah's extended detention, with authorities accusing him of breaking a law on demonstrations, inciting protesters to riot and block roads, and beating a police officer.

A law passed in November, which requires permits for all public gatherings, has angered human rights groups, especially since the military justified its removal of Morsi by saying it was responding to mass protests.