CAIRO Nine people were hurt when gunmen fired at protesters camping in Cairo's Tahrir Square on Tuesday, according to witnesses and Egyptian media, as the opposition called for a major demonstration it hopes will force President Mohamed Mursi to postpone a referendum on a new constitution.
Supporters of the Islamist leader, who want the vote to go ahead as planned on Saturday, were also gathering in the capital, setting the stage for further street confrontations in a political crisis that has divided the Arab world's most populous nation.
Police cars surrounded Tahrir Square in central Cairo, the first time they had appeared in the area since November 23, shortly after a decree by Mursi awarding himself sweeping temporary powers that touched off widespread protests.
The upheaval following the fall of Hosni Mubarak last year is causing concern in the West, in particular the United States, which has given Cairo billions of dollars in military and other aid since Egypt made peace with Israel in 1979.
The Tahrir Square attackers, some masked, also threw petrol bombs which started a small fire, witnesses said.
"The masked men came suddenly and attacked the protesters in Tahrir. The attack was meant to deter us and prevent us from protesting today. We oppose these terror tactics and will stage the biggest protest possible today," said John Gerges, a Christian Egyptian who described himself as a socialist.
The latest bout of unrest has so far claimed seven lives in clashes between the Islamist Muslim Brotherhood and opponents who are also besieging Mursi's presidential palace.
The elite Republican Guard which protects the palace has yet to use force to keep protesters away from the graffiti-daubed building, now ringed with tanks, barbed wire and concrete barricades.
The army has told all sides to resolve their differences through dialogue, saying it would not allow Egypt to enter a "dark tunnel". For the period of the referendum, the army has been granted police powers by Mursi, allowing it to arrest civilians.
The army has portrayed itself as the guarantor of the nation's security but so far it has shown no appetite for a return to the bruising front-line political role it played after the fall of Mubarak, which severely damaged its standing.
Leftists, liberals and other opposition groups have called for marches to the presidential palace later on Tuesday to protest against the hastily arranged constitutional referendum planned for Dec 15, which they say is polarising the country and could put it in a religious straightjacket.
Mohamed ElBaradei, a prominent opposition leader and Nobel prize winner, called for dialogue with Mursi and said the referendum should be postponed for a couple of months due to the chaotic situation.
"This revolution was not staged to replace one dictator with another," he said in an interview with CNN.
Outside the presidential palace, anti-Mursi protesters huddled together in front of their tents, warming themselves beside a bonfire in the winter air.
"The referendum must not take place. The constitution came after blood was spilt. This is not how a country should be run," said Ali Hassan, a man in his 20s.
Opposition leaders want the referendum to be delayed and hope they can get sufficiently large numbers of protesters on the streets to change Mursi's mind.
Islamists, who dominated the body that drew up the constitution, have urged their followers to turn out "in millions" in a show of support for the president and for a referendum they feel sure of winning.
Leftist politician Hamdeen Sabahy, one of the most prominent members of the National Salvation Front opposition coalition, said Mursi was driving a wedge between Egyptians and destroying prospects for consensus.
As well as pushing the early referendum, Mursi has angered opponents by taking extra powers he said were necessary to secure the transition to stability after the uprising that overthrew Mubarak 22 months ago.
"The road Mohamed Mursi is taking now does not create the possibility for national consensus," said Sabahy. He forecast polarisation if constitution were passed.
The National Salvation Front also includes ElBaradei and former Arab League chief Amr Moussa.
The opposition says the draft constitution fails to embrace the diversity of 83 million Egyptians, a tenth of whom are Christians, and invites Muslim clerics to influence lawmaking.
But debate over the details has largely given way to street protests and megaphone politics, keeping Egypt off balance and ill equipped to deal with a looming economic crisis.
Mahmoud Ghozlan, the Muslim Brotherhood's spokesman, said the opposition could stage protests, but should keep the peace.
"They are free to boycott, participate or say no; they can do what they want. The important thing is that it remains in a peaceful context to preserve the country's safety and security."
The disruption is also casting doubts on the government's ability to push through economic reforms that form part of a proposed $4.8 billion IMF loan agreement.
(Writing by Edmund Blair and Giles Elgood; Editing by Peter Graff and Janet McBride)