Thousands of backers and opponents of Egypt's Islamist president have held competing rallies in Cairo, the capital, and new clashes have erupted between the two sides in the country's second largest city, Alexandria.

The strife yesterday was a prelude to massive nationwide protests planned by the opposition over the weekend demanding President Mohammed Morsi's removal.

For the past several days, Mr Morsi's opponents and members of his Muslim Brotherhood have battled it out in the streets of several cities in the Nile Delta in violence that has left at least five dead..

Many fear the clashes are a sign of more widespread and bloodier battles to come tomorrow, the anniversary of Mr Morsi's inauguration, when the opposition says it will bring millions into the streets around the country.

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

Cairo International Airport was flooded with departures, in an exodus airport officials called unprecedented. They said all flights departing yesterday to Europe, the United States and the Gulf were fully booked with no vacant seats.

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.

In Alexandria yesterday, scuffles erupted between Mr Morsi's supporters and opponents, near the local headquarters of the Muslim Brotherhood. The fighting began when thousands of anti-Morsi protesters marched toward the HQ, where up to a 1000 supporters of the president were deployed, protecting the building. Someone on the Islamist side opened fire with on the marchers with a shotgun and the two sides began to scuffle.

Nine people were wounded by shot, Deputy Health Minister Mohammed al Sharkawi said. Security forces fired tear gas at the Brotherhood supporters, but when the two sides continued battling, they withdrew.

Each side insists it is and will remain peaceful tomorrow – and each has blamed the other for the violence so far.

Tamarod, the activist group whose anti-Morsi petition campaign evolved into tomorrow's planned protest, said in a statement it was 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. Tamarod says it has collected nearly 20 million signatures in the country of 90 million demanding Mr Morsi step down.

The Brotherhood says the five killed in the Delta clashes were its members. Some people "think they can topple a democratically elected president by killing his support groups," Gehad el Haddad, a Brotherhood spokesman, wrote on his Twitter account.

In Cairo, thousands of Morsi backers filled the street outside the Rabia el Adawiya Mosque, not far from the presidential palace. The palace, one of the sites where the opposition plans to hold rallies, has been surrounded by concrete walls.

In his Friday prayer sermon, the cleric of Rabia el Adawiya warned if Mr Morsi is ousted "there will be no president for the country" and Egypt will descend into "opposition hell". In the street, the Islamists chanted religious slogans. "It is for God, not for position or power," they shouted. "Raise your voice strong, Egyptian: Islamic Sharia." Many wore green headbands with the slogans of the Brotherhood.

Across the city, thousands of Morsi opponents massed in Cairo's central Tahrir Square, shouting for the president to "leave, leave".