Egyptians were on edge Saturday ahead of mass rallies against Islamist President Mohamed Morsi, as US President Barack Obama expressed concern after an American was killed during clashes between pro- and anti-government protesters.
Four people died and scores were hurt on Friday when Islamist supporters of Morsi turned out to challenge opponents demanding his departure, in what is seen as a prelude to possible confrontations on Sunday.
The army, which has stayed aloof from politics since Morsi was elected a year ago, has warned it will intervene if there is major unrest.
Speaking in South Africa, Obama said of Egypt: "We are all looking at the situation there with concern."
"We would urge all parties to make sure they are not engaging in violence and that police and military are showing appropriate restraint.
"Everybody has to denounce violence. We would like to see the opposition and President Morsi engage in a more constructive conversation about (how) to move their country forward."
Andrew Pochter, 21, an intern with US cultural and educational group AMIDEAST, was killed as he photographed clashes in the Mediterranean city of Alexandria, Egyptian officials said.
One man was killed during earlier clashes there, and another succumbed to his injuries on Saturday.
An Egyptian journalist in the Suez Canal city of Port Said was also killed and several others wounded by a small explosive device thrown at anti-Morsi protesters, a security official and witnesses said.
Across the country more than 130 people were reported wounded.
The offices of the Freedom and Justice Party, the political arm of the Muslim Brotherhood from which Morsi hails, were torched in Alexandria and at Aga in Daqahliya. Its offices were stormed in Beheira.
Washington warned Americans against travel to Egypt as antagonism intensifies between Morsi's supporters and the opposition, which accuses him of betraying the 2011 revolution that ousted Hosni Mubarak.
Britain urged its citizens to "avoid all demonstrations and large gatherings," while France said citizens should "limit movements to those strictly necessary".
Amnesty International called on the authorities to ensure the security forces showed restraint and protected peaceful demonstrators.
Morsi met his army chief Abdel Fattah al-Sisi and Interior Minister Mohammed Ibrahim "to discuss the final security plans to protect vital establishments," the official MENA news agency reported.
Morsi's opponents, a collection of leftists, liberals, Christians and also deeply religious Muslims, accuse him of hijacking the revolution and concentrating power in the hands of Islamist groups.
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Their call for nationwide protests on Sunday, demanding his resignation and snap elections, prompted pre-emptive demonstrations on Friday by the Muslim Brotherhood and its Islamist allies.
By Saturday, rival demonstrators had pitched tents and begun preparing for the long haul.
Morsi supporters spent the night outside the Rabaa Al-Adawiya mosque in Cairo's Nasr City neighbourhood, where tens of thousands gathered on Friday to defend the legitimacy of Egypt's first freely elected president.
"It's not just about Morsi, it's about legitimacy and the state. We can't go backwards," said protester Kamal Ahmed Kamel.
Others called on the opposition to invest their energy in the political process.
"If it is that big tomorrow, why can't they use the ballot box and participate in parliamentary elections and get rid of Morsi that way?" Kamel asked.
In Tahrir Square, epicentre of the 2011 revolt that toppled Mubarak, opponents also camped out and erected a large stage in preparation for what they call a "second revolution".
Some activists say they want Egypt governed by a presidential council and a national unity government.
"The Islamists have been in power for a year and they proved they failed at running the country," said one, Adel al-Amir.
Newspapers underlined the deep divisions in the Arab world's most populous nation.
Independent daily Al-Tahrir described Friday's rallies as "Egypt against the Brotherhood", while state-owned daily Al-Gomhuria talked of "The Battle of the Squares".
Morsi had warned in a televised speech on Wednesday that the growing polarisation threatened to "paralyse" Egypt, and appealed to the opposition to talk.
The opposition National Salvation Front coalition spurned his offer and renewed its demand for an election.
Sunday's protests have been called by Tamarod (Arabic for Rebellion), a grassroots movement which says it now has 22 million signatures on a petition demanding Morsi's resignation and a snap election.
Friday's pro-Morsi demonstrations were seen as a preemptive strike by his supporters against what they see as an attempt to subvert a fledgling democracy.
"We will not allow a coup against the president," senior Brotherhood leader Mohamed al-Beltagui told a mass rally in Cairo.