* Authorities order 15-day detention for deposed president
* Army chief says he wants mandate to confront violence
* Islamists warn of civil war, U.S. urges army restraint
* Clashes reported in Egypt's second city, Alexandria (Adds reports of death in Alexandria, security source on army)
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 opposing political camps took to the streets.
Confirming the potential for bloodshed, two men were killed in confrontations in Egypt's second city Alexandria and a further 19 were hurt, Mena news agency reported.
Heeding a call by army chief General Abdel Fattah al-Sisi for a popular mandate, hundreds of thousands rallied in numerous cities, welcoming the military's pledge to confront weeks of violence unleashed by the overthrow of Mursi on July 3.
Supporters of the deposed Islamist leader also staged mass, counter-demonstrations to demand his reinstatement, shrugging off fears of an imminent crackdown and vowing not to give in to an army demand for an immediate end to their protests.
A Reuters reporter said hundreds of people fought a pitched battled in Alexandria, with birdshot fired and men on roof tops hurling stones down at the crowds below. Seven protesters were also reported hurt in the Nile delta city of Damietta.
In Cairo, military helicopters repeatedly buzzed low over the main pro-Mursi tent vigil before wheeling around Tahrir Square, flooded with flag waving army supporters, some of whom held portraits of Sisi in full ceremonial uniform.
The West is increasingly concerned 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.
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 said he would now be detained for 15 days as a judge investigated a raft of allegations.
The probe centres on charges 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.
"GENERALS' FANTASY"
"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, which is close to army installations but far from Tahrir Square, focal point for the pro-army rally.
"It is either victory over the coup or martyrdom," said senior Brotherhood politician Mohamed El-Beltagy, addressing the packed pro-Mursi rally. "Our blood and our souls for Islam!" the crowds chanted.
The army has threatened to "turn its guns" on those who use violence, while the Brotherhood has warned of civil war, denying suggestions that it is provoking trouble.
Convulsed by political and economic turmoil, Egypt is deeply polarised, struggling to make the transition from the autocratic rule of the discredited Mubarak to free and open democracy.
"The Brothers stole our revolution," said Salah Saleh, a horse trainer at a central Cairo rally, voicing widespread criticism that Mursi refused to share out power after taking office and then failed to tackle Egypt's many woes.
"They came and sat on the throne and controlled everything."
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.
One security source said the military wanted to calm the situation after realising that Sisi's call to rally was not well received abroad. A military statement on Facebook said the protests did not represent a threat to the Brotherhood.
Youth leader Ahmed Maher, from the popular April Six youth movement, said his group would not join Friday's marches.
"Security forces do not need a mandate to tackle violence and terrorism," Maher said in a tweet.
"DAY OF LIBERATION"
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 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.
Helicopters dropped flyers at the pro-Mursi vigil calling on people to refrain from violence. The Brotherhood says it is the victim of unprovoked attacks, accusing "plain-clothes thugs" of kicking off the violence in Alexandria.
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 plunged.
He has appointed an interim government tasked with preparing for new parliamentary elections in about six months to be followed by a new presidential ballot.
The Tamarud ("Rebel") youth movement, which helped to 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 Kevin Liffey)
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