Updated 10:45 PM ET
CAIRO Tens of thousands of supporters and opponents of President Mohammed Morsi rallied 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 while at least five offices of Morsi's Muslim Brotherhood were torched, 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 Morsi's inauguration aimed at forcing his removal.
The opposition says it will bring millions into the streets across Egypt, and more violence is feared. Already, six people have been killed in clashes this week, including Friday's deaths.
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 because they were not authorized to talk to the press.
The U.S. State Department warned Americans against all but essential travel to Egypt, citing the uncertain security situation. It also said it would allow some nonessential staff and the families of personnel at the U.S. Embassy in Cairo to leave until conditions improve.
Opposition protesters in Alexandria broke into the local headquarters of Morsi's Muslim Brotherhood and set fires, throwing papers and furniture out the windows.
For several days, Brotherhood members and opponents of Morsi 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.
Morsi opponents massed in Cairo's Tahrir Square, the epicenter of the protests in 2011 that ousted longtime leader Hosni Mubarak. The crowd shouted, "Leave, leave" this time addressing Morsi. 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 neighborhood of Cairo, urging him to resign, Egypt's state news agency reported.
At the same time, tens of thousands of Morsi supporters, mainly Islamists, filled a public square outside the Rabia el-Adawiya Mosque, not far from the palace. Islamist parties have decided to hold a sit-in.
"They say the revolution is in Tahrir," said young activist Abdel Rahman Ezz, a Morsi 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.
In Alexandria, on the Mediterranean coast, fighting began when thousands of anti-Morsi demonstrators marched toward the Brotherhood's headquarters, where up to 1,000 supporters of the president were deployed, protecting the building.
When an unidentified person on 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. Online video posted by witnesses showed a protester carrying a gun who appeared to be shooting at the Brotherhood building.
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. State Department later confirmed the death, in a statement from Patrick Ventrell, a press office director.
"We are providing appropriate consular assistance from our Embassy in Cairo and our Bureau of Consular Affairs at the State Department," he said.
A medical official said the American died of gunshot wounds at a hospital. The official spoke on condition of anonymity because he was not authorized to speak to the press.
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 Morsi 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.
He accused thugs, remnants of the old regime, including members of Mubarak's disbanded National Democratic Party of being behind the attacks.
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.
Hundreds of protesters in the city of Bassioun threw stones at Freedom and Justice Party offices, tearing down the party sign.