By Tamim Elyan and Marwa Awad
CAIRO (Reuters) - Egypt's liberal and secular opposition has urged its supporters to vote down a divisive constitution put forward by Islamists, and set conditions for taking part in the referendum that will be hard for organizers to meet.
Islamist President Mohamed Mursi touched off a storm last month when he awarded himself sweeping powers to push through a hastily drafted basic law that he sees as an essential part of Egypt's transition to democracy.
The streets of Cairo and other cities have been the scene of often violent demonstrations by protesters opposed to a constitution they see as favorable towards the Islamists, and by pro-Mursi groups who back the document.
Seven people have died in the latest clashes between Muslim Brotherhood and opposition supporters. The presidential palace is surrounded by tanks, barbed wire and concrete barricades, although the army has not used force against protesters.
Efforts to resolve the worst political upheaval since the fall of Hosni Mubarak almost two years ago suffered a blow on Wednesday when the army called off planned "unity" talks involving rival factions, after an army official said the military was seen to be taking too political a role.
With the first round of polling in the Arab world's most populous nation only two days away, the opposition set out a list of demands for a fair vote that will test the organizers.
The opposition said it would still call for a boycott unless the referendum is held with full supervision by judges, security guarantees, and local and international monitoring. It also wants the vote held on one day rather than the two - December 15 and 22 - that the government has chosen.
State media said the two-day voting plan had been adopted because many of the judges needed to oversee the vote were staying away in protest at the decision to hold the referendum.
Voting therefore had to be staggered to move around those judges willing to cooperate, suggesting that it will be difficult to meet the opposition's demands for voting on only one day under full judicial supervision.
"We will vote 'no'," opposition politician and former Arab League chief Amr Moussa told Reuters.
Leftist Hamdeen Sabahy of the Popular Front added: "The Front decided to call on the people to take part in the referendum and reject this draft constitution and vote no.
"If these guarantees aren't in place by the day of the referendum on Saturday, we will withdraw from it."
Egyptians abroad began voting on Wednesday at embassies. At home, broadcast media gave procedural details, telling people they must dip a finger in ink after they cast their ballot to avoid multiple voting.
The absence of a boycott could help to ease confrontation on the streets and give the constitution more legitimacy if it passes.
The main opposition coalition says the draft constitution does not reflect the aspirations of all 83 million Egyptians because of provisions which could give Muslim clerics a role in shaping laws. It wants more safeguards for minority rights, including for the 10 percent of Egyptians who are Christian.
Mursi's supporters say the constitution is needed to continue the transition to democracy. Some deride their opponents as Mubarak-era "remnants" trying to cling to power.
Islamists have won parliamentary and presidential elections since the fall of Mubarak. They want the vote on the new constitution to go ahead and are confident it will pass, paving the way for them to win a new parliamentary election next year.
(Additional reporting by Yasmine Saleh and Edmund Blair in Cairo; Writing by Giles Elgood in Cairo; Editing by David Stamp)