Cairo - Egypt's vote on a divisive draft charter will take place in two rounds, its electoral commission said, after the army urged President Mohamed Morsi and his opponents to meet Wednesday over the crisis.
Cairo's presidential palace and surrounding streets were calm after a mass demonstration calling for Morsi's ouster, condemning the Muslim Brotherhood and opposing the draft constitution approved last month by an Islamist-dominated panel.
The charter has pitted Islamist allies of Morsi against secular-leaning foes in rival rallies that clashed last week, killing several people and wounding hundreds more.
The electoral commission decided to hold the vote in two separate, regional rounds on December 15 and 22, rather than one nationwide poll on Saturday, state television said.
The expatriate vote - itself postponed for days - began with the polling of more than 500,000 Egyptians at embassies and consulates in 150 countries, official news agency MENA said.
Armed forces chief and defence minister General Abdel Fattah al-Sissi on Tuesday called for Morsi and the opposition to meet as rival rallies numbering tens of thousands took to the streets in what are becoming daily mass protests.
Troops and tanks deployed on Tuesday outside the presidential palace in Cairo - the scene of the Egyptian capital's worst violence since before Morsi's election in June.
Protesters partially destroyed metal and concrete barriers a short distance away from the palace on Tuesday, pouring through to protest peacefully. There were no clashes, however.
Opposition protester Ahmed Fuad, 30, said: "If the majority turns out to vote, it'll be a no [for the constitution]. But we might be able to postpone the referendum."
A bigger Islamist counter-demonstration a few kilometres away gathered tens of thousands of referendum supporters whose mood was equally determined.
"It's the last battle for Islam against the secularists who want to ruin Egypt," said Ahmed Alaa, who was bussed in from the north of the country.
The military has said it fears the Arab world's most populous country is headed for a disastrous "dark tunnel" unless the two sides talk. It has warned it will not allow the situation to worsen.
The United States has urged Egypt's army, which it provides with $1.3bn each year, "to exercise restraint, to respect the right of peaceful protest".
The main opposition group, the National Salvation Front, has so far rejected talks with Morsi and his camp unless the referendum is scrapped. But it said it was weighing the army's appeal.
Morsi has previously declared himself ready to start dialogue with the Front but has said postponing the referendum is impossible.
Sissi said the proposed meeting for Wednesday, in a military sports complex in northeast Cairo, aimed to bring all political actors together along with youth movements, judges and journalists.
It would not, he said, be a forum for structured political negotiations but rather an attempt to come up with some sort of entente "for the sake of Egypt".
Real and legitimate questions
"We will not talk politics or the referendum. We will just sit together so that every Egyptian who is worried in their home is reassured," the armed forces chief said. "You can have differences, but not quarrel."
The opposition, made up of secular, leftwing and liberal groups, sees the draft constitution rushed through by the Islamist-dominated panel last month as weakening human rights, the rights of women and religious minorities.
Morsi's supporters argue it is up to voters to decide.
The United States said there were "real and legitimate questions" about the referendum process.
State Department spokesperson Victoria Nuland said there were fears for "public order surrounding the polling", but urged the military to show restraint.
The prolonged crisis is also intensifying economic uncertainty.
The International Monetary Fund on Tuesday put on hold a $4.8bn loan Egypt has sought to fill budget gaps it will face in the 2013-2014 fiscal year. The IMF had been expected to review the loan, which would have come with budget-cutting requirements attached, this month for final approval.