* Authorities order 15-day detention for deposed president
* Army chief says seeks mandate to confront violence
* Islamists warn of civil war, U.S. urges army restraint (Edits, adds quotes from rallies)
By Crispian Balmer and Asma Alsharif
CAIRO, July 26 (Reuters) - Ousted Egyptian president Mohamed Mursi is under investigation for an array of charges including murder, the state news agency said on Friday, stoking tensions as Egypt's opposing political camps took to the streets.
Army chief General Abdel Fattah al-Sisi has urged Egyptians to hold rallies to give the military a "mandate" to confront weeks of violence unleashed by his July 3 overthrow of Mursi. Supporters of the deposed Islamist have also announced protests.
Military officials have told Mursi's Muslim Brotherhood to end its protests and work with a new interim government or face the consequences, raising fears of a crackdown against Islamist protesters camped out by a mosque in a Cairo suburb.
Army helicopters buzzed low over Cairo shortly after noon prayers ended, and both sides warned of possible bloodshed. There were no reports of major violence by mid-afternoon.
Mursi has not been seen in public since his downfall and the army has said he is being held for his own safety. But Mena news agency said the former president would now be detained for 15 days as a judge investigated a raft of allegations.
The probe centres on charges that he conspired with Palestinian Islamist group Hamas to flee jail during the 2011 uprising against veteran autocrat Hosni Mubarak, killing some prisoners and officers, kidnapping soldiers and torching buildings.
Mursi has previously said locals helped him escape from prison during the 2011 upheavals and the Muslim Brotherhood denounced the series of accusations levelled against him.
"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."
Many thousands of men, women and children joined Brotherhood supporters at their round-the-clock vigil in northeast Cairo.
"Our blood and our souls for Islam!" the crowds chanted, showing no sign of backing down in their stand-off, which has polarised the Arab world's biggest state, long seen as crucial to the stability of the whole Middle East.
The army has threatened to "turn its guns" on those who use violence, while the Brotherhood has warned of civil war, denying suggestions it was provoking troubles.
A few hundred pro-army supporters gathered on Friday morning in Tahrir Square, centre of two years of turmoil in Egypt, before the main rally which might not peak until after the evening prayer marking the end of the day's Ramadan fast.
"The Brothers stole our revolution," said Salah Saleh, a horse trainer, voicing widespread criticism that Mursi and his allies refused to share out power when they took office and then failed to tackle Egypt's many economic and social woes.
"They came and sat on the throne and controlled everything."
Pro-army demonstrations were planned across Egypt, including the second city Alexandria, while Mursi's backers also announced widespread rallies, with 34 in the Cairo area alone.
"It is either victory over the coup or martyrdom," said senior Brotherhood politician Mohamed El-Beltagy, addressing the main pro-Mursi rally in the sun-baked Egyptian capital.
Confrontation appeared inevitable following a month of clashes in which close to 200 people, mainly supporters of Mursi, have been killed. Many Egyptians feared the worse.
"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 Mursi's overthrow constituted a coup, wording that would have triggered the cutoff of U.S. aid.
Mursi and other Brotherhood leaders were rounded up by the authorities during the 2011 revolt that 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 neighbouring Gaza Strip.
Hamas challenged investigators to find "one piece of evidence" that it had meddled in Egyptian affairs.
State Egyptian television screened images on Friday of the celebrations that erupted the night Sisi announced Mursi had been deposed. The narrator declared it "the day of liberation from the Brotherhood occupation".
"Egypt against terrorism," declared a slogan on the screen.
Witnesses said army helicopters had dropped flyers at the pro-Mursi vigil calling on people to refrain from violence. The Brotherhood says it is the authorities themselves who have stirred up violence to justify a looming crackdown.
Army chief 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 powerful military, but Sisi turned against him after a year in which the economy floundered and support for the Brotherhood fell.
He has appointed an interim government tasked with preparing for fresh parliamentary elections in about six months to be followed by a new presidential ballot.
The Tamarud ("Rebel") youth movement that helped rally millions in anti-Mursi street protests before the army moved against him, stressed its support for Sisi on Friday.
"The Tamarud movement affirms the mandate given by the Egyptian people to the armed forces to carry out the law with speed, justice and resolve," it said in a statement. (Additional reporting by Shadia Nasralla, Asma Alsharif, Tom Perry, Noah Browning, Tom Finn, Maggie Fick, Omar Fahmy, Edmund Blair, Mark Felsenthal, Matt Spetalnick and Michelle Nichols; Writing by Crispian Balmer and Matt Robinson; Editing by Michael Georgy and Mike Collett-White)