* Pro- and anti-government factions to make their case
* Final attempt to gather support before referendum
* Vote on constitution is prelude to national elections
By Yasmine Saleh
CAIRO, Dec 14 (Reuters) - Flag-waving supporters of Egyptian President Mohamed Mursi staged a final rally on Friday before a divisive referendum on a new constitution that the Islamist leader hopes will bring an end to weeks of political crisis and street clashes.
Cairo and other cities have seen a series of often violent demonstrations over the past three weeks since Mursi assumed sweeping new powers to push through the constitution, which he sees as a vital element of Egypt's transition to democracy after the overthrow of autocratic predecessor Hosni Mubarak last year.
At least eight people have died and hundreds have been injured, and a leading opposition figure warned of more blood on the streets during the voting this Saturday and next on a draft the opposition says is too heavily influenced by Islamists.
The referendum, held on two days because there aren't enough judges willing to monitor all polling stations, asks Egyptians to accept or reject a basic law that must be in place before national elections can be held early next year - an event many hope can steer the Arab world's most populous nation towards stability.
To bolster support for the constitution, Islamists who propelled Mursi to power in June's presidential election assembled at a mosque near the president's palace in Cairo.
"We've come here to say 'yes' to the constitution," they chanted, "Long live President Mursi." The majority of protesters were men with beards, and some had brought their children and veiled wives along.
"I came to say 'yes' to the legitimacy of President Mursi and to Islamic Sharia law," said Mohamed Murad, 37, a preacher at a mosque.
Members of the liberal, secular and Christian opposition began to gather to protest against the basic law outside the presidential palace.
Mohamed ElBaradei, an opposition leader and Nobel prize winner, urged Mursi to cancel the referendum "before it is too late".
Amr Moussa, a former head of the Arab League also prominent in the opposition, called on Egyptians to vote "no".
The measure is nevertheless expected to pass, given the well-organised Muslim Brotherhood's record of winning elections since the fall of Mubarak. Many Egyptians, tired of turmoil, may simply fall in line.
The first round of voting on Saturday will take place in Cairo and other major cities. Official results won't be announced until after the second round, though it is likely that details will emerge to give a good steer on the first-day figures, which are expected to show a strong vote in favour.
To provide security for the vote, the army has been deployed in force, with state television showing ranks of soldiers receiving their orders to protect polling stations and other government buildings. About 120,000 troops and 6,000 tanks and armoured vehicles will be deployed.
While the military backed Mubarak and his predecessors, it has not intervened on either side in the present crisis.
The vote has proved hugely controversial, with supporters of the Islamist Muslim Brotherhood fighting on the streets with opposition groups.
The opposition says the constitution does not reflect the aspirations of all 83 million Egyptians because it is too Islamist and tramples on minority rights, including those of the Christian community. Mursi's supporters say the constitution is needed if progress is to be made towards democracy.
While the opposition is telling its supporters to vote "no", it has also threatened to boycott proceedings if guarantees for a fair vote are not met. But staying away from the process could risk a loss of credibility, political experts say.
Many ordinary Egyptians are well aware of how contentious the constitution is, but simply want to get it out of the way so the country can move ahead to a more stable future.
"Do I like the constitution?" asked Ahmed Helmy, a 35-year-old engineer in Tahrir Square. "No. But I want the referendum to take place so we can get out of this prolonged transitional period that's making me and millions of Egyptians wish we had left the country."