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At least 51 protestors and three members of security forces were killed and 435 people were injured Monday in bloody clashes outside a military building in Cairo, as thousands continued to demand change in Egypt’s leadership.

Mohammed Sultan, head of Egypt’s emergency services confirmed the death toll and said the victims were wounded mostly by live ammunition and birdshot.

Eyewitnesses tell Fox News that Muslim Brotherhood supporters of ousted President Mohammed Morsi were fired upon with birdshot and rounds from automatic weapons at around 5:30 a.m. local time, during morning prayers. Fox News sources who were at the scene shortly after the shooting described seeing Muslim Brotherhood supporters with large chest wounds and fatal gunshot wounds in their backs.

The Muslim Brotherhood have blamed the attacks on the Egyptian military who were standing guard over the demonstration near to the Raba El Adwyia mosque.

Monday’s fighting  caused the highest number of fatalities since massive protests forced Morsi’s government from power last week and established an interim civilian administration.

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    The interim administration issued a statement on the state news agency Monday expressing regret for the bloodshed in Cairo, according to Reuters. The statement said the violence was the result of an attempt by protesters to attack the Republican Guard building and asked protesters not to approach any military “or other vital installations.” Transitional administration officials also said they formed a judicial committee to investigate the incident.

    There were conflicting reports on how the fighting began, with Morsi’s supporters saying it was an unprovoked attack and the military saying they came under assault first.

    At a nationally televised press conference Monday afternoon, spokesmen for the military and police said troops guarding the republican Guard complex came under “heavy gunfire” in the middle of the night as attackers stationed on rooftops opened fire with guns and molotov cocktails.

    A spokesman confirmed that one army officer and two policemen were dead, and 42 were injured in the raid.  The military says it had a right to defend the building, adding that the protest “was no longer peaceful.”  The spokesman also pointed out that suspected Islamists have coordinated armed attacks on several military facilities recently in the Sinai Peninsula.

    One witness, university student Mirna el-Helbawi, also said gunmen loyal to Morsi opened fire first, including from the roof of a nearby mosque. El-Helbawi, 21, lives in an apartment overlooking the scene.

    Supporters of Morsi, however, said the security forces fired on hundreds of protesters, including women and children, at the sit-in encampment as they performed early morning prayers.

    The military blamed the attacks on so-called "terrorist" groups. Late Sunday night military helicopters dropped leaflets on pro-Morsi protesters in the Cairo university area calling on Morsi followers and terrorists to end their protests.

    Meanwhile, Egypt's top Muslim cleric -- Sheik Ahmed el-Tayeb-- warned of "civil war" and announced he was going into seclusion as a show of protest to both sides until the violence ceases.

    El-Tayeb, the head of the Al-Azhar Mosque, said he had "no choice" but to seclude himself at home "until everyone shoulders his responsibility to stop the bloodshed instead of dragging the country into civil war."

    A presidency spokesman said Monday’s violence will not derail efforts to form a new interim government. “What happened will not stop steps to form a government, or the (political) roadmap,” said Ahmed Elmoslmany, according to a Reuters report.

    Al Nour, Egypt's second-most prominent Islamist party after the Muslim Brotherhood, has said it will withdraw from negotiations with the interim government and the Muslim Brotherhood has called for its supporters to "rise up" against those who want to steal the revolution.

    At a news conference in Washington Monday, White House press secretary Jay Carney said halting aid immediately to Egypt wouldn't be in the best interests of the U.S. Carney told reporters the U.S. is still reviewing whether the Egyptian military's ouster of Morsi should be labeled a coup d'etat. Under American law, that label would force the U.S. to stop sending aid to Egypt.

    ``We are going to take the time necessary to review what has taken place and to monitor efforts by Egyptian authorities to forge an inclusive and democratic way forward,'' Carney said.

    Carney also said the White House will consult with Congress and any decision about what to call Morsi's ouster will be in line with U.S. policy objectives.

    The Egyptian military-- which effectively supported the anti-Morsi movement-- now may face pressure to impose stricter security measures to try to keep unrest from spilling out of control. It will also have to produce compelling evidence to support its version of events or otherwise suffer what is already shaping to be a Brotherhood media blitz to portray the military as a brutal institution with little regard for human life or democratic values.

    In a move that is likely to further inflame the situation, the Freedom and Justice party, the Muslim Brotherhood's political arm, called on Egyptians to rise up against the army. Morsi has been a longtime leader of the Brotherhood.

    Liberal politician Mohamed ElBaradei expressed dismay at the bloodshed, sending a message on Twitter that read "Violence begets violence and should be strongly condemned. Independent Investigation a must. Peaceful transition is only way."

    Satellite broadcaster Al-Jazeera showed footage from a nearby field hospital of at least six dead bodies laid out on the ground, some with severe wounds. A medic from the area, Hesham Agami, said ambulances were unable to transport more than 200 wounded to hospitals because the military had blocked off the roads.

    Al-Shaimaa Younes, who was at the sit-in, said military troops and police forces opened fire on the protesters during early morning prayers. "They opened fire with live ammunition and lobbed tear gas," she said by telephone. "There was panic and people started running. I saw people fall."

    Women and children had been among the protesters, she said.

    Morsi supporters have been holding rallies and a sit-in outside the Republican Guard building since the military deposed Morsi last week during massive protests against him. The military chief replaced Morsi with an interim president, until presidential elections are held. But Morsi's supporters refuse to recognize the change in leadership and insist Morsi be reinstated. Besides the Republican Guard sit-in, they are also holding thousands-strong daily rallies at a nearby mosque.

    Morsi's opponents are also holding rival rallies. They say the former president lost his legitimacy by mismanaging the country and not ruling democratically, leading to a mass revolt that called on the army to push him from office.

    Military spokesman Col. Ahmed Mohammed Ali said initial information indicates that gunmen affiliated with the Brotherhood tried to storm the Republican Guard building shortly after dawn, firing live ammunition and throwing firebombs from a nearby mosque and rooftops. One police officer on the scene was killed, he said. Another military spokesman, who declined to be named because he was not authorized to brief reporters, said five from the Brotherhood side were killed.

    A statement by the armed forces published on the state news agency said "an armed terrorist group" tried to storm the Republican Guard building, killing one officer and seriously injuring six. The statement said the forces arrested 200 attackers, armed with guns and ammunition.

    After declaring the ouster of Morsi last Wednesday, the Defense Minister Abdel-Fattah el-Sissi replaced him with Egypt's chief justice and suspended the constitution until new presidential elections.

    The Associated Press and Reuters contributed to this report.