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, 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.
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.
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.
"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. 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-Morsi 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 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.
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-Morsi 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 Morsi 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, traveling 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.
Tamarod, the activist group whose anti-Morsi petition campaign evolved into Sunday's protest, said in a statement it opposed "to any attack against anybody, whatever the disagreement with this person was," and accused the Brotherhood of sparking violence to scare people from participating Sunday.
Tamarod says it has collected nearly 20 million signatures in the country of 90 million demanding Morsi step down.
"We are against Morsi because he does not govern in the name of the Egyptian people, but in the name of the Brotherhood group," said Ayed Shawqi, a teacher at an anti-Morsi rally in Alexandria.
Outside the Rabia el-Adawiya Mosque, the pro-Morsi crowd waved Egyptian flags while speakers addressed them from a stage. A banner proclaimed, "Support legitimacy," the slogan Morsi's supporters have adopted, arguing that protests must not be allowed to overturn an elected president.
They also waved the Brotherhood's flag -- a green banner with two swords -- and carried Morsi posters and portraits.
"This is a revolution, and there is no other one!" they chanted. Speakers onstage praised the military and the crowd responded with, "The army and the people are one hand," seeking to keep the military on the side of the president.
"Those who burn and those who kill are the traitors of this nation," Brotherhood preacher Safwat Hegazi told the crowd. "Mr. President, use a heavier hand, your kind heart won't be any use. ... We want to complete our revolution and purify our country."
Assem Abdel-Maged, leader of the formerly militant Gamaa Islamiya group, threatened to "sever heads" of opposition supporters if they attacked the military. Rafai Taha, one of the leading figures of Gamaa Islamiya, was also onstage, next to Brotherhood leaders.
In his Friday sermon, the cleric of Rabia el-Adawiya warned that if Morsi is ousted, "there will be no president for the country," and Egypt will descend into "opposition hell."
Pro-Morsi marchers -- many wearing green headbands with the slogans of the Muslim Brotherhood -- chanted religious slogans. "It is for God, not for position or power!" they shouted. "Raise your voice high, Egyptian: Islamic Shariah!"
The anti-Morsi demonstrators in Tahrir Square also waved Egyptian flags. They cheered, clapped, whistled and chanted, "Egypt, Egypt, Egypt. Long live Egypt!" and "The people want the fall of the regime," a phrase heard repeatedly in 2011.
One banner depicted President Barack Obama and said, "Obama supports terrorism."