CAIRO (AFP) - Egypt's constitution-drafting panel voted on Sunday 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 following the ouster of Islamist president Mohamed Morsi in July.

More than 2,000 pro-Mursi students meanwhile poured out of university campuses and managed to reach Cairo's Tahrir Square, where they held the biggest Islamist demonstration in the iconic roundabout - the epicentre of the 2011 revolt that toppled long-ruling president Hosni Mubarak - since Mursi's ouster.

Police moved in to disperse the protesters with tear gas, the violence underscoring the country's lingering polarisation nearly four months after the military removed Mursi from power following a turbulent year in office.

Mursi, Egypt's first democratically elected president, was overthrown by the military on July 3, and in mid-August the security forces launched a sweeping crackdown on his supporters which has left more than 1,000 people killed and thousands more jailed.

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.

The thorny issue of the insular military's long-standing 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.

Another top activist, Ahmed Maher, was freed on Sunday after he turned himself in at a Cairo court on Saturday following an order for his arrest.

A law passed earlier this month, which requires permits for all public gatherings, has angered secular and human rights groups, especially since the military justified its removal of Mursi by saying it was responding to mass protests.

On Sunday, the army came in for more criticism from Human Rights Watch, which accused it of "forcibly disappearing" five top Mursi aides since his ouster.

Article 234 stipulates that the defence minister be appointed in agreement with the Supreme Council of the Armed Forces, although panel spokesman Mohammed Salmawy told AFP this clause will only apply during the first two presidential terms.

The constitution would also keep the military's budget beyond civilian scrutiny.

Army chief and defence minister General Abdel Fattah al-Sisi - appointed by Mursi in August 2012 - is hugely popular and seen as the real power behind the interim government after he led Morsi's ouster.

Referendum "the real test"

Political analyst Hassan Nafea said secular Egyptians would be angry over the constitutional provisions concerning the army.

"That will trigger debates among the secular camp at a time when the new protest law has already angered them," the professor of political science at Cairo University told AFP.

Mr Nafea said the referendum - which would be followed by presidential and parliamentary elections in mid-2014 - will be the real test of the charter.

"I am not sure the constitution would be passed with a big majority by the Egyptian people... when (the) nation is polarised. People will not vote on the basis of whether the constitution is good or bad, but... on the basis of which camp you belong to," he said.

The interim authorities suspended the constitution written under Mursi after his removal on July 3. That charter had been drafted by a 100-member panel dominated by his Islamist allies.

The current panel includes representatives from civil society, political parties, institutions including the army and police, and the Coptic church.

It has just two Islamists, neither of whom is from Mursi's Muslim Brotherhood, which won a series of polls after Mubarak's ouster.