Called out by the army, some of the largest crowds to date in Egypt's 2½ years of upheaval took to the streets across the country Friday.

An Egyptian health ministry official said seven people were killed in clashes in the coastal city of Alexandria. Demonstrations in Cairo remained mostly peaceful into the night.

Egyptian army chief Gen. Abdel-Fattah el-Sissi called for rallies in support of a proposed mandate to stop "violence and terrorism," raising speculation he may be planning a crackdown on protests in support of Mohammed Morsi, the first democratically elected president who was removed on July 3 by el-Sissi.

Security around the pro-army demonstrations has been heavy after el-Sissi vowed to protect the rallies from attacks by rivals. Tanks guarded one entrance to Tahrir Square and a heavy police presence guarded other entrances.

Morsi's supporters also held rallies Friday, but in vastly smaller numbers.

Both sides tried to show how much public support they enjoy, but the millions who turned out for the pro-army demonstrations overwhelmed the streets in multiple cities.

Friction started outside a major mosque in Alexandria, as the two sides exchanged stones and fired birdshot. Police and the army tried to break up the fighting, lobbing tear gas and deploying soldiers.

Morsi in secret detention

A criminal investigation into allegations Morsi conspired with the Palestinian militant group Hamas to break himself and about 30 other members of his Muslim Brotherhood party out of a prison outside Cairo during the 2011 uprising against autocrat Hosni Mubarak fuelled tensions Friday.

The former president has spent three weeks in secret detention where military intelligence agents have extensively questioned him on the inner workings of his presidency and of the Brotherhood, military officials told The Associated Press.

Throughout, Morsi has been denied access to television and newspapers, they said. He has been moved at least three times between Defence Ministry facilities in armoured vehicles under heavy guard. He is currently in a facility outside Cairo, they said, without elaborating.

On Friday, a spokesman for Morsi's Muslim Brotherhood said the move to prosecute Morsi showed "the complete bankruptcy of the leaders of the bloody coup."

Egyptians "reject the return of the dictatorial police state and all the repression, tyranny and theft it entails," Ahmed Aref said in a statement.

The news of the investigation, which is likely to pave the way to a formal indictment, was the first word on Morsi's legal status since the military deposed him on July 3.

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Supporters and opponents of Egypt's ousted president Mohammed Morsi clash in Alexandria on Friday. At least two people were killed and about two dozen injured in protests in that city, but clashes took place across Egypt. (Tarek Elframawy/Associated Press)

Next steps uncertain

It remains unclear what steps the military is planning after Friday's show of public strength. The most explosive step would be if it were to try to break up sit-ins by Islamists who have been camped out at locations in Cairo and other cities for weeks.

The military also could move to arrest more than a dozen Brotherhood figures who have arrest warrants against them. Or it could take firmer action to stop any sign of violence by Islamist protesters — though the Morsi camp says it is the one targeted by attacks.

Clashes have repeatedly erupted the past three weeks pitting Morsi supporters against his opponents or security forces. Each side blames the other for sparking violence, and people in both camps have been seen carrying weapons.

At the same time, Islamic militants have stepped up attacks on the army and police in the Sinai Peninsula.

Nearly 200 people have been killed since Morsi's fall.

Hamas denies prison break help

The prosecutors' announcement on Morsi could signal a greater move to go after the Brotherhood in courts. Besides Morsi, five other senior figures from the group have been detained.

Over recent months, a court in the Suez Canal city of Ismailia has heard testimonies from prison officials and intelligence officers indicating that Morsi and his Brotherhood colleagues were freed when gunmen led by Hamas operatives stormed the Wadi el-Natroun prison. At least 14 members of the security forces were killed and the jail's documents and archives destroyed.

Muslim Brotherhood officials have said they got out when local residents broke into the prison to free their relatives and that they had no knowledge ahead of time of the prison break.

Hamas has consistently denied any involvement. On Friday a spokesman for the militant group, Sami Abu Zuhri, condemned Morsi's detention order.

"The Egyptian decision is an attempt to drag Hamas into the Egyptian conflict," he said. "We call on the Arab League to bear its responsibility in confronting the incitement against Hamas."

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Gen. Abdel-Fattah el-Sissi led the ouster of Egypt's democratically elected president, Mohammed Morsi. (Jim Watson, Pool/Associated Press)

Morsi still 'legitimate' president

Morsi's only account of his jailbreak came in a frantic phone call he made to Al-Jazeera Mubasher TV moments after being freed. "From the noises we heard ... It seemed to us there were (prisoners) attempting to get out of their cells and break out into the prison yard, and the prison authorities were trying to regain control and fired tear gas," Morsi said in the call.

By the time they got out, the prison was empty, and there was no sign of a major battle, he said.

Senior Brotherhood official Essam el-Erian rejected the detention order, saying Morsi continues to enjoy immunity as the nation's "legitimate" president, and he can stand trial only as part of a constitutional process that allows that.

The detention order, he wrote on his official Facebook page, "lays bare the fascist nature of military rule ... our response will be with millions in peaceful rallies in the squares."