* Army chief says seeks mandate to confront violence
* Gives Brotherhood deadline to join roadmap-source
* Islamists warn of civil war, U.S. urges army restraint
* U.S. won't rule on whether there was a coup (Adds quotes, newspaper headline)
By Yasmine Saleh and Matt Robinson
CAIRO, July 26 (Reuters) - A deeply polarised Egypt bracedfor bloodshed on Friday in rival mass rallies summoned by thearmy that ousted the state's first freely elected president andby the Islamists who back him.
Both sides warned of a decisive struggle for the future ofthe Arab world's most populous country, convulsed by politicaland economic turmoil since the 2011 uprising that ended 30 yearsof autocratic rule by Hosni Mubarak.
Army chief General Abdel Fattah al-Sisi has called Egyptiansinto the streets nationwide to give the military a "mandate" toconfront weeks of violence unleashed by his July 3 overthrow ofIslamist President Mohamed Mursi.
A military official said the army had given Mursi's MuslimBrotherhood a Saturday deadline to end its resistance and join amilitary-set road map to fresh elections, signalling a turningpoint in the confrontation.
The Brotherhood fears a crackdown to wipe out an Islamistmovement that emerged from decades in the shadows to win everyelection since Mubarak's fall but was brought down by the armyafter barely a year in government.
The movement, which has manned a street vigil for almost amonth with thousands of followers demanding Mursi's return, hascalled its own counter-demonstrations. Confrontation appearedinevitable following a month of clashes in which close to 200people, mainly supporters of Mursi, have died.
The army threatened to "turn its guns" on those who useviolence. The Brotherhood warned of civil war.
"We will not initiate any move, but will definitely reactharshly against any calls for violence or black terrorism fromBrotherhood leaders or their supporters," the army official toldReuters.
The streets of Cairo were largely empty early Friday. Crowdswere not expected to gather until later in the day, and mightnot peak until after the evening prayer marking the end of theday's Ramadan fast. Many locals feared the worst.
"I'm staying home all day, it's too dangerous to work. Ididn't think things in Egypt could get this bad, but every dayyou hear about clashes and deaths," said Shadi Mohamed, a22-year-old taxi driver. "Egypt is a disaster."
There is deepening alarm in the West over the course takenby the country of 84 million people, a pivotal nation betweenthe Middle East and North Africa and recipient of some $1.5billion a year in mainly military aid from the United States.
Signalling its displeasure at recent events, Washington saidthis week it had delayed delivery of four F-16 fighter jets toCairo and called on the Egyptian army on Thursday to exercise"maximum restraint and caution" during Friday's rallies.
Deputy Secretary of State William Burns, however, said onThursday the Obama administration did not intend to rule onwhether Mursi's overthrow constituted a coup, wording that wouldtrigger the cutoff of U.S. aid.
"We believe that the continued provision of assistance toEgypt, consistent with our law, is important to our goal ofadvancing a responsible transition to democratic governance," hetold members of Congress.
Brotherhood supporters have been camped out in a Cairosquare since June 28, guarded by men with sticks behindbarricades and sandbags. They fear a repeat of the July 8killing of more than 50 Mursi supporters when security forcesopened fire outside a Cairo barracks.
The Brotherhood says it wants nothing to do with the army'stransition plan. With Mursi still in military detention at anundisclosed 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 helpedrally millions in anti-Mursi street protests before the armymoved against him.
"We will not let extremists ruin our revolution," he said.
The Interior Ministry said it would undertake "unprecedentedmeasures" to protect people and property. The Al-Gomhouriagovernment newspaper carried a front page headline in bold red:"On the Friday of the mandate: we are Egypt's soldiers".
Witnesses said army helicopters had dropped flyers at theBrotherhood vigil calling on people to refrain from violence. The Brotherhood says it is the authorities themselves who havestirred up violence to justify their crackdown.
Sisi delivered his call on Wednesday in full militaryuniform and dark sunglasses. He was appointed by Mursi in a bidby the president to rein in Egypt's all-powerful military, butSisi turned against him after a year in which the Egyptianeconomy floundered and support for Mursi slumped.
Posters of the general have since appeared in shops andstalls across Cairo and were handed out overnight to drivers instreets leading to Tahrir square.
The country remains deeply split over what happened on July3. The Brotherhood accuses the army of ejecting a democraticallyelected leader in a long-planned coup, while its opponents saythe army responded to the will of the people.
Sisi announced the nationwide rallies after a bomb attack ona police station in Mansoura, a city north of Cairo, in which apoliceman was killed.
Since Mursi was deposed, hardline Islamists have alsoescalated a violent campaign against the state in the lawlessSinai Peninsula, near Egypt's border with Israel and thePalestinian Gaza Strip, with daily attacks on security forces.
The influential Sheikh Ahmed al-Tayeb, head of Egypt's topIslamic institute Al-Azhar, urged Egyptians to heed the army'scall. "The Azhar's understanding is that the army's protest callwas 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." (Additional reporting by Shadia Nasralla, Asma Alsharif, TomPerry, Noah Browning, Tom Finn, Maggie Fick, Omar Fahmy, MarkFelsenthal, Matt Spetalnick and Michelle Nichols; Writing byMatt Robinson; Editing by Crispian Balmer)