Two Egyptians were killed in clashes between supporters and opponents of ousted President Mohamed Morsi in Egypt's second biggest city of Alexandria on Friday, Egypt's MENA news agency reported.
It said 19 others were injured in clashes that erupted after thousands took to the streets in response to a call by army chief General Abdel Fattah al-Sisi for a popular mandate to tackle violence unleashed since the ouster of Morsi on July 3. That triggered calls for pro-Morsi rallies.
Seven protesters were also reported hurt during clashes in the Nile delta city of Damietta.
Tens of thousands of demonstrators gathered for a mass protest to support the army in Tahrir square in Cairo.
Clashes between opponents of Morsi and his supporters also erupted in a Cairo neighborhood on Friday afternoon, AFP reported.
AFP cited footage from Egyptian ONTV as showing demonstrators hurling stones at each other in the capital city's neighborhood of Shubra.
A few hundred pro-army supporters gathered early Friday morning in Tahrir Square, center of two years of turmoil in Egypt, before the main rally which could possibly peak after the evening prayer marking the end of the day's Ramadan fast.
Armored personnel carriers guarded every entrance to the square.
The Brotherhood, which has manned a street vigil for almost a month with thousands of followers demanding Morsi's return, has called for its own counter-demonstrations.
Confrontation appeared inevitable following a month of clashes in which close to 200 people, mainly supporters of Morsi, have been killed. Many people in the Arab world's most populous country feared the worst.
The Egyptian army on Friday also said it was detaining Morsi over accusations of kidnapping, killing soldiers and other charges, according to the state news agency.
Both sides have warned of the potential for bloodshed in Egypt, which has been convulsed by political and economic turmoil since the 2011 uprising that ended 30 years of autocratic rule by the US-backed Hosni Mubarak.
State news agency Mena said the mooted charges against Morsi included conspiring with the Palestinian Islamist group Hamas, killing prisoners and officers "deliberately with prior intent", kidnapping officers and soldiers, and setting fire to the prison of Wadi el-Natroun.
They relate to his escape from the prison in 2011, when he was arrested during the uprising against Mubarak, and provide legal grounds for his continued detention.
Morsi has been held by the military since the army ousted him from office on July 3 following huge street protests against his troubled, one-year rule. Washington has previously called for him to be freed.
His Muslim Brotherhood denounced news of the accusations.
"At the end of the day we know all of these charges are nothing more than the fantasy of a few army generals and a military dictatorship," Brotherhood spokesman Gehad El-Haddad said. "We are continuing our protests on the streets."
Throwing down the gauntlet to the Brotherhood, army chief General Abdel Fattah al-Sisi has called on Egyptians to rally nationwide on Friday to give the military a "mandate" to confront weeks of violence unleashed by Morsi's removal.
A military official said the army had given Morsi's Muslim Brotherhood a Saturday deadline to end its resistance and join a military-set road map to new elections, signalling a turning point in the confrontation.
The Brotherhood fears a crackdown to wipe out an Islamist movement that emerged from decades in the shadows to win every election since Mubarak's fall but then struggled to tackle Egypt's growing economic and social woes.CIVIL WAR
The army has threatened to "turn its guns" on those who use violence. The Brotherhood warned of civil war.
"We will not initiate any move, but will definitely react harshly against any calls for violence or black terrorism from Brotherhood leaders or their supporters," an army official told Reuters.
"The people, the army and the police are one hand," shouted a policeman, leading a group of chanters.
"I'm staying home all day, it's too dangerous to work. I didn't think things in Egypt could get this bad, but every day you hear about clashes and deaths," said Shadi Mohamed, a 22-year-old taxi driver. "Egypt is a disaster."
There is deepening alarm in the West over the course taken by the country of 84 million people, a pivotal nation between the Middle East and North Africa and recipient of $1.5 billion a year in mainly military aid from the United States.
Signalling its displeasure at recent events, Washington said this week it had delayed delivery of four F-16 fighter jets to Cairo and called on the Egyptian army to exercise "maximum restraint and caution" during Friday's rallies.
Deputy Secretary of State William Burns, however, said on Thursday the Obama administration did not intend to rule on whether Morsi's overthrow constituted a coup, wording that would have triggered the cutoff of US aid.CONFUSION
Morsi and many other Brotherhood leaders were rounded up by the authorities during the 2011 uprising that eventually swept Mubarak from power.
Many managed to escape in the ensuing confusion, alongside militants from Hamas, a Palestinian offshoot of the Muslim Brotherhood that governs in the neighboring Gaza Strip.
Mursi and his fellow Brotherhood members have said they were freed by local residents from the Wadi el-Natroun prison. State news agency Mena said investigating judge Hassan Samir had already confronted Morsi with evidence during questioning.
Brotherhood supporters have been camped out in a Cairo square since June 28, guarded by men with sticks behind barricades and sandbags. They fear a repeat of the July 8 killing of more than 50 Morsi supporters when security forces opened fire outside a Cairo barracks.
Witnesses said army helicopters had dropped flyers at the vigil calling on people to refrain from violence. The Brotherhood says it is the authorities themselves who have stirred up violence to justify their crackdown.
Sisi delivered his call for rallies on Wednesday in full military uniform and dark sunglasses. He was appointed by Mursi in a bid by the president to rein in Egypt's all-powerful military, but Sisi turned against him after a year in which the economy floundered and support for the Brotherhood fell.