* Fears of violence as rival camps mobilise for rallies
* Interim PM cites fear of casualties
* Brotherhood says aims for biggest protest yet (Recasts with army statement, ultimatum, White House)
By Yasmine Saleh and Asma Alsharif
CAIRO, July 25 (Reuters) - Egypt's army threatened onThursday to turn its guns on those who use violence, itsstarkest warning yet ahead of what both sides expect will be abloody showdown in the streets between supporters and opponentsof deposed president Mohamed Mursi.
An army official said the military had issued an ultimatumto Mursi's Muslim Brotherhood, giving the Islamist group untilSaturday to sign up to a plan for political reconciliation whichit has so far spurned.
The army has summoned Egyptians into the streets for Fridayand made clear it intends the day to mark a turning point in itsconfrontation with the followers of Mursi, the elected leaderthe generals removed on July 3.
Mursi's Brotherhood, which has maintained a street vigil fora month with thousands of followers demanding Mursi's return,has called its own crowds out for counter-demonstrations acrossthe country in a "day to remove the coup".
Both sides have dramatically escalated rhetoric ahead ofFriday's demonstrations. The Brotherhood accused the army ofpushing the nation towards civil war and committing a crimeworse than destroying Islam's holiest site.
The army issued its warning in a statement posted on aFacebook page. It will not "turn its guns against its people,"the statement said, "but it will turn them against blackviolence and terrorism which has no religion or nation".
A military official said the army had given the Brotherhood48 hours from Thursday afternoon to join the political process.He did not reveal what the consequences would be if theBrotherhood refuses.
Army chief General Abdel Fattah al-Sisi has called onEgyptians to take to the streets and give him a "mandate" totake action against the violence that has convulsed Egypt sincehe shunted its first freely elected president from power.
The Brotherhood, an Islamist movement that won repeatedelections since the fall of autocrat Hosni Mubarak in 2011, saysit is the authorities themselves that have stirred up theviolence to justify their crackdown.
The main anti-Mursi youth protest group, which has ralliedbehind the army, said its supporters were taking to the streetsto "cleanse Egypt".
The West is increasingly alarmed at the course taken byEgypt, a strategic hinge between the Middle East and NorthAfrica, since the Arab Spring protests brought down Mubarak andended decades of autocratic rule.
Signalling its displeasure, Washington has delayed deliveryof four F-16 fighter jets to Cairo. On Thursday, the White Houseurged the army to exercise "maximum restraint and caution".
The United States has yet to decide whether to call themilitary's takeover a "coup", language that would require it tohalt $1.5 billion it sends in annual aid, mostly for the army.
"CLEANSE EGYPT"
For weeks, the authorities have rounded up some Brotherhoodofficials but tolerated the movement's presence on the streets,with thousands of people attending its vigil demanding Mursi'sreturn and tens of thousands appearing at its demonstrations.
That patience seems to have run out. Prime Minister Hazemel-Beblawi, head of the interim cabinet installed by the army,said there was escalating violence by increasingly well-armedprotesters, citing a bomb attack on a police station.
"The presence of weapons, intimidation, fear - this causesconcern, especially when there are calls for many to come outtomorrow from different sides," he told a news conference.
After a month in which close to 200 people have died inviolence triggered by Mursi's downfall, many fear the protestswill lead to more bloodshed.
Past incidents of violence have tended to run through thenight and into the following day. Another security officialforecast violence beginning Friday night and stretching intoSaturday, the period covered by the army's ultimatum. He alsoindicated that the two-day period was expected to be decisive.
"The history of Egypt will be written on those days," saidthe official, part of a security establishment that accuses theIslamists of turning to violence.
The Brotherhood blames the violence on the authorities,accusing them of stirring it up to justify a crackdown with theultimate goal of wiping the group out.
Reiterating his group's commitment to peaceful protest,senior Brotherhood politician Farid Ismail accused the securityservices of readying militias to attack Mursi supporters, addingthat Sisi aimed to drag Egypt into civil war.
"His definition of terrorism is anyone who disagrees withhim," Ismail told Reuters. "We are moving forward in completepeacefulness, going forward to confront this coup."
Muslim Brotherhood leader Mohamed Badie issued a statementaccusing Sisi of committing a crime worse than destroying theKabaa - the site in Mecca to which all Muslims face when theypray - "brick by brick".
But many Egyptians are no less passionately backing thearmy, determined to see the Brotherhood reined in.
"There are men carrying guns on the street ... We will notlet extremists ruin our revolution," said Mohammed Abdul Aziz, aspokesman for Tamarud, an anti-Mursi petition campaign thatmobilised protests against his rule.
"Tomorrow we will cleanse Egypt," he told Reuters.
UNCOMPROMISING
Sisi's speech on Wednesday pointed to the deepeningconfrontation between the Brotherhood and the militaryestablishment, which has reasserted its role at the heart ofgovernment even as it says it aims to steer clear of politics.
Saying it moved against Mursi in response to the biggestpopular protests in Egypt's history, the army installed aninterim cabinet that plans to hold parliamentary elections inabout six months, to be followed by a presidential vote.
The Brotherhood says it wants nothing to do with thetransition plan. With Mursi still in military detention at anundisclosed location, there is slim hope for compromise.
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. The government said it was a terroristattack. The Brotherhood also condemned the bombing, accusing theestablishment of seeking to frame it.
Since Mursi was deposed, hardline Islamist groups have alsoescalated a violent campaign against the state in the lawlessSinai Peninsula, with daily attacks on the police and army.
Two more soldiers were killed on Thursday in an attack on acheckpoint, security and medical sources told Reuters.
At the Brotherhood protest camp in front of a Cairo mosque,Mursi supporters said they expected the army to provoke violenceto justify its crackdown.
"The army itself will strike. They will use thugs and thepolice," said Sarah Ahmad, a 24-year-old medical student.
Essam El-Erian, another senior Brotherhood politician,accused "the putschists" of trying to recreate a police state,telling a televised news conference: "This state will neverreturn, and Egypt will not go backwards." (Additional reporting by Tom Perry and Maggie Fick, NoahBrowning, Tom Finn and Shadia Nasralla; Writing by Tom Perry andMatt Robinson; Editing by Peter Graff)
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