A U.S. citizen was stabbed to death during clashes in Alexandria on Friday, a doctor and three security officials said. One Egyptian man was also shot dead, and dozens were injured in the clashes between supporters and opponents of President Mohammed Morsi in Egypt's second largest city.

A third man was killed and 10 injured in an explosion during a protest in Port Said, at the mouth of the Suez Canal. Police on Saturday said the cause was unclear but protesters, believing it was a bomb, attacked an Islamist party office in the city.

Egypt's leading religious authority warned of "civil war" after violence in the past week that had already left several dead and hundreds injured. They backed President Mohammed Morsi's offer to talk to opposition groups ahead of the mass demonstrations planned for Sunday, that the opposition hopes can force the Islamist president to quit.

The United Nations, European Union and United States have appealed for restraint and urged Egypt's deadlocked political leaders to step back from a confrontation threatening the new democracy that emerged from the Arab Spring revolution of 2011.

The U.S. embassy said in a statement it was evacuating non-essential staff and family members and renewed a warning to Americans not to travel to Egypt unless they had to.

The Muslim Brotherhood said eight of its offices had been attacked on Friday, including the one in Alexandria. Officials said more than 70 people had been injured in the clashes in the city. One was shot dead and a young American man who was  recording the events by camera died after being stabbed in the chest.

A member of the ruling Muslim Brotherhood was also shot dead overnight in the city of Zagazig. Six Egyptians have been killed in the recent days of clashes. 

The violence erupted when anti-Morsi protesters tried to storm offices used by Mursi's Muslim Brotherhood in Alexandria, a city on Egypt's Mediterranean coast. The American .

Friday's demonstrations were called in advance of a day of mass marching on Sunday, when President Mohammed Morsi's critics hope millions will hit the streets to demand new elections.

"Vigilance is required to ensure we do not slide into civil war," said clerics of Cairo's thousand-year-old Al-Azhar institute, one of the most influential centers of scholarship in the Muslim world.

In a statement broadly supportive of Morsi, it urged dialogue and blamed "criminal gangs" who besieged mosques for violence which the Brotherhood said has killed at least five supporters in a week.

The Brotherhood's political wing warned of "dire consequences that will pull the country into a violent spiral of anarchy". It held liberal leaders, including former UN diplomat Mohamed ElBaradei, personally responsible for inciting violence by hired "thugs" once employed by the ousted dictator, Hosni Mubarak.

Opposition leaders condemned the violence. The army, which has warned it could intervene if political leaders lose control, issued a statement saying it had deployed across the country to protect citizens and installations of national importance.

An Islamist rally in Cairo included calls to reconciliation.

In Alexandria, Egypt's second city, at least 70 people were wounded, many by birdshot, officials said. Nine policemen were also injured after hundreds fought around a Brotherhood office.

As several thousand anti-Morsi protesters marched along the Mediterranean seafront, a Reuters reporter saw about a dozen men throw rocks at guards outside the building. They responded. Bricks and bottles flew. Gunshots went off. Eventually, the office was trashed and documents burned, watched by jubilant youths chanting against the country's Islamist leaders.

Days of political violence in Nile Delta towns between Cairo and Alexandria continued. More than 40 were wounded on Friday in two towns. Overnight, the Brotherhood said, one man was killed and four were wounded in a raid on its office in Zagazig.

Cairo relatively calm

There was no trouble during the afternoon in Cairo when tens of thousands of Islamists gathered round a mosque after weekly prayers to show support for Morsi. His opponents hope millions will turn out on Sunday to demand he step down, a year to the day since he was sworn in as Egypt's first freely chosen leader.

"I came to support the legitimate order," said Ahmed al-Maghrabi, 37, a shopkeeper from the Nile Delta city of Mansoura whose hand bore grazes from street fighting there this week. "I am with the elected president. He needs to see out his term."

There was a mostly festive atmosphere in the hot sunshine.

Some speakers reflected fear among Islamists that opponents aim to suppress them as Mubarak did. But there was also talk from the podium of the need for dialogue - a concern also of international powers worried by bitter polarisation.

At one point, a song was played praising unity among "Muslims and Christians, Islamists and liberals" - a marked contrast to a similar gathering in the same spot last Friday when hardliners warned opponents against attacking Morsi.

Standing above pictures of those killed, Abdel Rahman al-Barr, a Brotherhood leader, said: "The only way forward is for us to sit down together ... To those who smash a hole in the ship of state, we will not respond by smashing another. We will work to repair the hole. We will not let the ship sink."

Some opposition gatherings were also under way, though small, perhaps due to mixed messages from leaders about whether to start the "June 30" protest movement early. A handful of protesters watched security men ringing the presidential palace, the focus for Sunday's Cairo rally. Morsi has moved elsewhere.

A few thousand milled around in the capital's Tahrir Square, cradle of the revolution. Some waved red cards reading "Out!", in preparation for the big demonstration against the president.

A protest of about 3,000 in Port Said, a bastion of anti-Islamist sentiment on the Suez Canal, passed off peacefully. Shipping in the international waterway was unaffected.

Strategic

The army, which heeded mass protests in early 2011 to push aside Mubarak, has warned it will intervene again if there is violence and to defend the "will of the people." Both sides believe that means the military may support their positions.

The United States, which funds Egypt's army as it did under Mubarak, has urged compromise and respect for election results. Egypt's 84 million people, control of the Suez Canal and treaty with Israel all contribute to its global strategic importance.

UN chief Ban Ki-moon urged Egyptians to respect "universal principles of peaceful dialogue" and to strengthen their democracy by promoting an "inclusive environment."

The European Union's foreign policy chief Catherine Ashton urged all sides to keep protests peaceful, build trust and show a "spirit of dialogue and tolerance."

In Alexandria, opposition marchers said they feared the Brotherhood was usurping the revolution to entrench its power and Islamic law. Others had economic grievances, among them huge lines for fuel caused by supply problems and panic buying.

"I've nothing to do with politics, but with the state we're in now, even a stone would cry out," said 42-year-old accountant Mohamed Abdel Latif. "There are no services, we can't find diesel or gasoline. We elected Mursi, but this is enough.
"Let him make way for someone else who can fix it."

It is hard to gauge how many may turn out on Sunday but much of the population, even those sympathetic to Islamic ideas, are frustrated by economic slump and many blame the government.

Previous protest movements since the fall of Mubarak have failed to gather momentum, however, among a population anxious for stability and fearful of further economic hardship