CAIRO - A deeply polarized Egypt braced for bloodshed on Friday in rival mass rallies summoned by the army that ousted the state's first freely elected president and by the Islamists who back him.Both sides warned of a decisive struggle for the future of the Arab world's most populous country, convulsed by political and economic turmoil since the 2011 uprising that ended 30 years of autocratic rule by Hosni Mubarak.Army chief General Abdel Fattah al-Sisi has called Egyptians into the streets to give the military a "mandate" to confront weeks of violence unleashed by his July 3 overthrow of Islamist President Mohamed Morsi. 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 fresh 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 was brought down by the army after barely a year in government.The movement, which has manned a street vigil for almost a month with thousands of followers demanding Morsi's return, has called its own counter-demonstrations. Confrontation appeared inevitable following a month of clashes in which close to 200 people, mainly supporters of Morsi, have died.The army 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," the army official told Reuters. 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 some $1.5 billion yearly in aid from the United States, mainly for the military.Signalling its displeasure, Washington said this week it had delayed delivery of four F-16 fighter jets to Cairo and called on the Egyptian army on Thursday to exercise "maximum restraint and caution" during Friday's rallies."This is a critical time for Egyptians to come together, particularly if they want to move beyond cycles of unrest and instability," White House spokesman Josh Earnest told reporters aboard Air Force One."WE WILL CLEANSE EGYPT" 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.The Brotherhood says it wants nothing to do with the army's transition plan. With Mursi still in military detention at an undisclosed location, there is slim hope for compromise."Tomorrow we will cleanse Egypt," said Mohammed Abdul Aziz, a spokesman for the Tamarud ("Rebel") youth movement that helped rally millions in anti-Morsi street protests before the army moved against him. "There are men carrying guns on the street," he told Reuters. "We will not let extremists ruin our revolution." Anti-Morsi protesters began gathering overnight in Cairo's Tahrir square, epicentre of the rallies that brought down Mubarak and preceded the army's overthrow of Morsi, as well as at the capital's presidential palace, in Egypt's second city of Alexandria and in Port Said on the Suez canal.The rallies were expected to peak after the evening prayer marking the end of the day's Ramadan fast.Witnesses said army helicopters had dropped flyers at the Brotherhood vigil calling on people to refrain from violence. The Interior Ministry said it would undertake "unprecedented measures to protect citizens and their property".The Brotherhood says it is the authorities themselves that have stirred up the violence to justify their crackdown.Sisi delivered his call on Wednesday in full military uniform and dark sunglasses. He was appointed by Morsi 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 Egyptian economy floundered and support for Morsi slumped.Posters of the general have since appeared in shops and stalls across Cairo.The country remains deeply split over what happened on July 3. The Brotherhood accuses the army of ejecting a democratically elected leader in a long-planned coup, while its opponents say the army responded to the will of the people.Sisi announced the nationwide rallies after a bomb attack on a police station in Mansoura, a city north of Cairo, in which a policeman was killed.Since Morsi was deposed, hardline Islamists have also escalated a violent campaign against the state in the lawless Sinai Peninsula, near Egypt's border with Israel and the Palestinian Gaza strip, with daily attacks on security forces.The influential Sheikh Ahmed al-Tayeb, head of Egypt's top Islamic institute Al-Azhar, urged Egyptians to heed the army's call."The Azhar's understanding is that the army's protest call was made for all Egyptians to unite and stand against violence," he said in a statement aired on state television. "I ask all Egyptians to rally to save Egypt."