Tens of thousands of supporters and opponents of President Mohamed Morsy rallied on Friday in Cairo, and both sides fought each other in the second-largest city of Alexandria, where two people were killed including an American and 85 were injured, officials said.

The competing camps were trying to show their strength before even bigger nationwide protests planned by the opposition Sunday the first anniversary of Mr. Morsy’s inauguration aimed at forcing his removal.

The opposition says it will bring millions into the streets across Egypt, and more violence is feared.

The Cairo International Airport was flooded with departing passengers, an exodus that officials said was unprecedented. All flights departing Friday to Europe, the U.S. and the Gulf were fully booked, they said.

Many of those leaving were families of Egyptian officials and businessmen and those of foreign and Arab League diplomats as well as many Egyptian Christians, the officials said, speaking on condition of anonymity.

Opposition protesters in Alexandria broke into the local headquarters of Mr. Morsy’s Muslim Brotherhood and set fires, throwing papers and furniture out the windows.

For several days, Brotherhood members and opponents of Mr. Morsy have battled in cities in the Nile Delta. With Friday’s deaths, at least six have been killed this week.

“We must be alert lest we slide into a civil war that does not differentiate between supporters and opponents,” warned Sheik Hassan al-Shafie, a senior cleric at Al-Azhar, the country’s most eminent Muslim religious institution.

Mr. Morsy opponents massed in Cairo’s Tahrir Square, the epicentre of the protests in 2011 that ousted long time leader Hosni Mubarak. The crowd shouted, “Leave, leave” this time addressing Mr. Morsy. Tents were put up on the grass in the middle of the historic square.

Dozens of protesters also gathered at the gates of the presidential palace in the Heliopolis neighbourhood of Cairo, urging him to resign, Egypt’s state news agency reported.

At the same time, tens of thousands of Morsy supporters, mainly Islamists, filled a public square outside the Rabia el-Adawiya Mosque, not far from the palace.

“They say the revolution is in Tahrir,” said young activist Abdel Rahman Ezz, a Morsy supporter who addressed the crowd. “It is true the revolution started in Tahrir. But shamefully, today the remnants of the old regime are in Tahrir. The revolutionary youth are here.”

The palace is one of the sites where the opposition plans to gather Sunday and has been surrounded by concrete walls. Islamist parties have decided to hold a sit-in at nearby Rabia el-Adawiya.

In Alexandria, on the Mediterranean coast, fighting began when thousands of anti-Morsy demonstrators marched toward the Brotherhood’s headquarters, where up to 1,000 supporters of the president were deployed, protecting the building.

Someone on the Islamist side opened fire with birdshot on the marchers, and the melee erupted, according to an Associated Press cameraman. Security forces fired tear gas at the Brotherhood supporters, but when the two sides continued battling, they withdrew. Protesters later broke into the building and began to trash it.

Alexandria security chief Gen. Amin Ezz Eddin told Al-Jazeera TV that an American was killed in Sidi Gabr Square while photographing the battle. The U.S. Embassy told The Associated Press it was trying to confirm the report.

A medical official said the American died of gunshot wounds at a hospital.

The Alexandria health department reported an Egyptian also died from a gunshot wound to the head. It was not immediately known if that victim was a Morsy opponent or supporter.

The country witnessed a wave of attacks against Muslim Brotherhood offices across the country. The Brotherhood’s media spokesman, Gehad el-Haddad, said on his Twitter account that eight of his group’s headquarters were attacked and looted, and two were burned down.

Much of the violence was in the provinces of the Nile Delta, north of Cairo.

Protesters stormed an office of the Brotherhood, attacked members inside, injuring 10, and set the office on fire in the city of Shubrakheit, the state news agency said. Others stormed a Brotherhood office in the coastal city of Baltim, destroying electronic equipment, and another of the group’s branches was torched in the city of Aga.

The Brotherhood says at least five of those killed this week were its members. Some people “think they can topple a democratically elected President by killing his support groups,” el-Haddad said earlier on his Twitter account.

There were reports of violence from the Islamist side in the Delta as well.

At least six people were injured when an anti-Morsy march was attacked by the president’s supporters in the city of Samanod, according to a security official. Attackers fired gunshots and threw acid at the protesters as they passed the house of a local Brotherhood leader, the official said.

In the city of Tanta, four men believed to be Morsy supporters tried to attack a mosque preacher during his sermon, in which he called on worshippers to stand with Al-Azhar’s calls to avoid bloodshed.

In Qalioubia, north of Cairo, “popular committees” charged with managing traffic stopped a caravan of more than 90 Islamists heading to Cairo, according to a security official. The group, travelling in a bus and three minibuses, carried Molotov cocktails, clubs and gas cans, the official said.

One small bus escaped, but the others were turned over to police, the official said, speaking on condition of anonymity because he was not allowed to talk with the press.

Each side has insisted it is peaceful and will remain so Sunday, blaming the other for violence.

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