Egyptian security forces fired teargas in Cairo's Tahrir Square to disperse anti-government protesters on Sunday, as a new constitution that reinforces the military's political power edged closer to approval.

The draft constitution reflects how the balance of power has shifted in Egypt since secular-minded generals deposed Islamist President Mohamed Mursi in July after mass protests against him. It contains language that could ban Islamist parties outright.

It opens the way for a presidential election to be held before parliamentary polls, potentially changing the transition plan outlined by the army when it ousted Mursi.

The original plan said a parliamentary election should take place before the presidential one. But the draft constitution avoids stipulating which vote should happen first.

The draft constitution says the "election procedures" must start within six months from the date of the constitution's ratification, meaning Egypt may not have an elected president or parliament until the second half of next year.

The change was announced by former Arab League secretary general Amr Moussa, chairman of the 50-member constituent assembly, as it completed its final draft on Sunday. The draft must now be put to a referendum this month or next.

A major milestone in Egypt's political roadmap, the constitution must be approved in a referendum before new elections, which Mursi's Muslim Brotherhood, driven underground by security measures and a legal ban, is unlikely to contest.

"The people want to topple the regime," chanted several hundred protesters who descended on Tahrir Square, epicentre of the 2011 uprising against autocratic President Hosni Mubarak.

Though it only lasted about half an hour before security forces acted, it appeared to be the biggest protest by Brotherhood sympathisers in Tahrir since Mursi's fall. "With our blood and souls we sacrifice for you, Islam," chanted some.

One scaled a lamp post where he hung a picture of Mursi. Others flashed the four-finger hand sign denoting sympathy with the hundreds of Mursi supporters shot dead by the security forces when they broke up their Cairo sit-ins on August 14.

Some of the protesters said they were not from the Brotherhood. "I want Sisi out and a real end to army rule," said Ramez Ibrahim, 32, a professor of political science, referring to armed forces chief General Abdel Fattah al-Sisi.

Army vehicles moved in to drive the demonstrators away and later sealed off the square completely. Some passersby shouted abuse at the protesters, others waved in support. Earlier, protesters set a police truck ablaze near Cairo University.

The government says it is determined to implement a law passed last week that heavily restricts protests. Criticised by the United States, the law has hardened fears of pro-democracy campaigners about the future of political freedoms in Egypt.

One of two leading secular activists detained for calling protests in defiance of the law was released on Sunday.

The 2011 Tahrir Square uprising awoke hopes of a new era of freedom in the most populous Arab nation. But three years of turmoil have made many Egyptians yearn for stability.

Sisi is seen as an army strongman and a front-runner for the presidency, though he has yet to declare his candidacy.

Mursi's fall set off the bloodiest bout of internal strife in Egypt's modern history, with hundreds of his partisans killed and armed attacks on the security forces becoming commonplace.

Some 200 policemen and soldiers have been killed in what the military-backed government casts as a war on terrorism. The Brotherhood says it is peacefully resisting the army takeover.

A few hundred metres from Tahrir Square, the 50-member constituent assembly was voting on the draft constitution whose provisions include a ban on parties formed on a religious basis. Islamists have won all post-Mubarak national votes.

The assembly, chaired by Moussa, voted against five of the document's 247 articles, including one requiring parliamentary polls to be held before a presidential vote.

Some assembly members had called for presidential elections to be held before parliamentary ones, citing the weakness of secular political parties. The assembly, which is working to a December 3 deadline, was to redraft the provision later on Sunday.

The draft constitution widens the already broad privileges enjoyed by the army by requiring the approval of the Supreme Council of the Armed Forces for the choice of a defence minister to serve for eight years from when the document is ratified.

It does not indicate how the minister of defence could be sacked or who has the authority to fire him.

The new constitution will replace one drafted by an Islamist-dominated assembly and signed into law by Mursi last year after it was approved in a referendum. The new text strips out Islamist-inspired additions introduced last year.

The Nour Party, an ultra-conservative Islamist party that backed Mursi's ouster, has described the draft as "satisfying".

But one Mursi ally, Gamaa Islamiya leader Assem Abdel Maged, said it was a "constitution of minorities", and that the army had driven Egypt to the "edge of a precipice".-Reuters