Egypt's mainly secular opposition gathered for last-ditch protests on Friday against a new constitution it slams as divisive, but on the eve of a referendum it held back from urging a boycott.

The opposition National Salvation Front expressed "deep concern" over the vote called by Islamist President Mohamed Morsi, which is intended to complete the transition from veteran strongman Hosni Mubarak's rule.

But while again protesting against the spreading of polling over two successive Saturdays, the opposition refrained from urging supporters to stay away from a plebiscite that analysts say the Islamists are likely to carry.

The bloc has organised weeks of protests against what it regards as the hijacking of the pro-democracy uprising against Mubarak by an Islamist-dominated panel to push through a constitution that leans too far towards Islamic sharia law.

Clashes last week between opposition protesters and Morsi supporters left eight people dead and hundreds injured.

Mohamed ElBaradei, a former UN nuclear energy agency chief who heads the opposition Front, said on his Twitter feed: "Insistence on referendum in an explosive, polarized, chaotic & lawless environment is leading country to the brink."

Morsi has ordered Egypt's military to help police maintain security until the results of the referendum are known. A total of 130,000 police and 120,000 soldiers will be deployed, interior ministry and military officials said.

Morsi spread polling in the referendum over two days because of a lack of judges willing to provide the statutory supervision for the vote.

Polls will open on Saturday in the biggest cities of Cairo and Alexandria and eight other provinces. A week later, on December 22, the other half of the country will vote.

Morsi's Muslim Brotherhood and ultra-orthodox Salafist groups backing the draft charter have been campaigning for days for Egypt's 51 million voters to approve it.

Hundreds of them gathered early Friday to back a "yes" vote. Its supporters are stationed along main roads holding "Yes to the constitution" placards.

The opposition -- which initially wanted the referendum postponed and has said it could yet call a last-minute boycott if the poll is insufficiently monitored by judges and independent observers -- has only started urging a "no" vote since Thursday.

"It's you who will pay the price if you vote 'yes.' 'No' to the constitution," said an online campaign advertisement by an opposition group called April 6.

At least two anti-constitution rallies were to be held in Cairo on Friday, after the main weekly Muslim prayers.

International watchdogs, including the UN human rights chief, as well as the United States and the European Union, have expressed reservations about the draft constitution because of loopholes that could be used to weaken human rights, the rights of women and the independence of judges.

Analysts said the proven ability of the Muslim Brotherhood to get its voters out meant it was likely -- but not certain -- the draft constitution would pass.

If it is adopted, "it will exacerbate political tension and result in more acrimony," Hani Sabra, an Egypt specialist for the Eurasia Group, said in an evaluation note.

"The Brotherhood and Morsi believe that if the constitution is adopted, it represents a mandate to pursue their policies. However, majority support does not translate into the bureaucracy falling in line. It will also not silence the opposition," he wrote.

Polls open on Saturday at 8:00 am (0600 GMT) and are scheduled to close at 7:00 pm (1700 GMT).