Mohamed Askar, a senior military spokesman, said the protesters "came at us with machine guns, with live rounds, with bird shot." Askar said Egyptian troops were shot at from nearby rooftops and that one officer was killed with a bullet that struck him on the top of the head. Another soldier was wounded and shown in video taken by the military, his chest peppered with shotgun pellets.
Protesters described a scene of confusion and chaos, as live gunfire, bird shot and tear gas seemed to come from all sides.
"I don't remember where we were facing, but the shooting came from everywhere," said Abdel Rahman Mahmoud, a young subway cleaner, who sat on the concrete at the Brotherhood's makeshift field hospital, both arms bandaged and sweat beading on his forehead.
Abdel Naguib Mahmoud, a lawyer from the Nile Delta town of Zagazig, said he and fellow protesters had knelt to the pavement for the second time, their backs to the Republican Guard palace, when he heard shouted warnings from the perimeter that security forces were encroaching.
"So we finished our prayer rapidly," Mahmoud said. He said he heard the resounding boom of tear gas canisters being fired and the crackle of gunfire. Running toward the entrance of the sit-in area, he and several friends began to pick up the wounded, Mahmoud said. More shots rang out, and the men lay down on the pavement.
Mahmoud said he saw forces in military fatigues and police dressed in black. Moments later, an officer stood over him and kicked him, telling him to move, he said. When he ran, gunmen opened fire. He said he was hit in the back with birdshot, and he lifted his shirt to reveal a scattering of small bloodied wounds.
The violence dealt a significant blow to an already fragile political process. The Brotherhood's Freedom and Justice party issued a statement calling for an "uprising against those who want to steal the revolution with tanks" and asking the world to prevent a "new Syria."
The ultra-conservative Salafist Nour party, the only Islamist group to support Morsi's ouster, said it would abandon negotiations over who should take over as prime minister of Egypt to protest what it described as a "massacre."
Sheikh Ahmed el-Tayeb of the al-Azhar Mosque, Egypt's top Islamic authority, had previously expressed support for Morsi's ouster. But on Monday, he appeared on state television and called for all political prisoners to be freed and for a transition period back to democracy of no more than six months. He said he would remain in seclusion at his home "until everybody takes responsibility to stop the bloodshed, to prevent the country from being dragged into a civil war."
Meanwhile, the main Tamarod activist group, which organized the massive protests last week that led to Morsi's removal, called for the Brotherhood's political wing to be dissolved and its leadership barred from political life.
That treatment, Tamarod said on Twitter, would echo the ban placed on former president Hosni Mubarak's political party after the 2011 Egyptian revolution. A ban on the Brotherhood and other religious parties would also fall in line with Mubarak's own policy, under which many of the Brotherhood's leaders spent decades moving in and out of prison.
Askar, the senior military spokesman, suggested that the soldiers guarding the Republican Guard palace were targeted in a coordinated attack that involved rifle fire, molotov cocktails and positions atop nearby tall buildings.
He said 200 people were arrested during the day and that the investigations and interrogations would be done by police and prosecutors, not by the army.
Askar disputed statements, videos and evidence offered by Muslim Brotherhood activists, charging that bullet casings, for example, could not have traveled hundreds of yards from soldiers' guns into the crowds. "I will tell you where they came from," he said. "The protesters had these bullets with them."
Seeking to explain their version of events, military and police spokesmen showed reporters images from army cameras on the ground and aboard helicopters, as well as some news footage, saying the images portrayed an increasingly fierce attack on troops by Muslim Brotherhood supporters.
Individuals in the crowd are shown hurling rocks at the troops and later launching shards of toilet bowls from rooftops and throwing what appear to be spears. Tires are set ablaze, and one group of young men is shown filling bottles and throwing molotov cocktails.
The army evidence includes videos that show one man with a short rifle and another with a handgun firing at the soldiers.
"Any law in the world allows soldiers to defend Egyptian security when confronted with live fire," another military spokesman, Ahmed Mohamed Ali, told reporters. "We are no longer talking about peaceful protests."
At an emotional news conference at the Rabaa al-Adawiya Mosque, where Morsi supporters have camped since the Islamist president was deposed on Wednesday, a doctor and others said that protesters had been shot in the back as they knelt to pray.
"These past three or four hours have been the worst in my life," said Hisham Ibrahim, the doctor who is directing the field hospital outside the mosque that had received many of the victims. He said the makeshift medical center was equipped only for routine first aid and lacked the supplies to handle a mass shooting.
The field hospital had been set up in a building adjacent to the mosque, but its supplies and triage center spilled onto the concrete surface outside the building. Volunteers tied tarpaulins overhead to create shade, and dirty mats were placed on the ground for patients to lie upon. Medics huddled over a man with the deep gash of a bullet wound in his right thigh, while their colleagues treated a man who had been shot in the arm.
".Bloodbath," a Muslim Brotherhood official, Gehad el-Haddad, said on Twitter.
Morsi was forced from office last week by Egypt's powerful military, which said it was motivated to act by millions of anti-government demonstrators who had taken to the streets to demand that Morsi leave.
Since his ouster, however, Morsi supporters have turned out in force, triggering clashes with security forces on Friday. While the weekend was largely quiet, Monday's violence ratcheted up the tension considerably and made the goal of forming some sort of national unity government appear ever more elusive.
The U.S. Embassy in Cairo announced Monday that it would be closed to the public on Tuesday, citing the risk of protests near the embassy compound, which is in the center of Cairo close to Tahrir Square. Many anti-Morsi demonstrations have taken on an anti-American tone, with protesters asserting that the Obama administration supports the Muslim Brotherhood. The Obama administration has been cautious in its comments about the coup, urging a peaceful transition back to democratic elections.
Over the weekend, negotiations were snarled in a dispute over what role, if any, should be given to Mohamed ElBaradei, a Nobel Peace Prize winner, former diplomat and liberal politician who is supported by many liberal and secular members of the anti-Morsi movement, but whom ultra-conservative Islamists deeply distrust.
ElBaradei on Monday denounced the violence outside the Guard headquarters, saying on Twitter: "violence begets violence and should be strongly condemned. Independent Investigation a must. Peaceful transition is only way."
Hours before the shooting, hundreds of Morsi supporters began a separate standoff with the military outside Egypt's Defense Ministry in eastern Cairo.
Mohsen Radi, a former member of parliament from the Brotherhood's Freedom and Justice party who marched to the defense compound with a crowd of about 1,000 people, said the group was expanding its peaceful sit-ins to amplify the "pressure" to reinstate Morsi, Egypt's first democratically elected president.
Outside the Defense Ministry, the Morsi supporters formed a human wall, their arms linked, in a face-off about 100 yards from a row of armored personnel carriers and army troops at the ministry's gate. As one protester unfurled a large banner, featuring an image of Morsi, onto the pavement in front of the Brotherhood supporters, an army officer said over a loudspeaker: "If you move one more meter, you will be shot."
A third, even larger sit-in has been underway outside the Rabaa al-Adawiya mosque in eastern Cairo for more than a week. The demonstration has been the epicenter of pro-Morsi protests. Demonstrators, including families and children from across the country, have set up a a sprawling encampment there.
By early afternoon, Morsi supporters outside the Republican Guard headquarters building had erected fortifications built of paving stones that stood nearly six feet tall. The protesters shouted, "Come down you traitors!" at soldiers on a nearby rooftop, who were themselves reinforcing their positions with sandbags.
Fresh bullet holes were visible in the doors of cars and metal light posts. Men wearing blood-splattered clothes milled through the crowd, posing for pictures. The protesters marked - with bottles, bricks and tree branches - the spots on the pavement where the dead or wounded had left behind pools of sticky drying blood.
Near the protesters' barricades, a six-story building was burning, smoke billowing out of the top floors.
Amro Hassan and Sharaf al-Hourani in Cairo contributed to this report.
U.S. won't cut off aid to Egypt's military
Washington • The Obama administration signaled Monday the importance of continued aid to the Egyptian military, which overthrew the elected President Mohammed Morsi last week. The White House and State Department both said the military would not be punished with a cutoff of its $1.3 billion in annual U.S. aid for toppling Morsi. But if the U.S. government makes a legal determination that the removal was done through a coup d'etat, U.S. law would require ending all non-humanitarian aid to Egypt, the vast majority of which goes to the military.
Administration officials said lawyers were still reviewing developments to make that ruling. But the absence of a coup determination, coupled with the administration's refusal to condemn Morsi's ouster, sent an implicit message of U.S. approval to the military.
Officials said the White House had made clear in U.S. inter-agency discussions as recently as a Monday morning conference call that continued aid to Egypt's military was a priority for U.S. national security, Israel's safety and broader stability in the turbulent Middle East that should not be jeopardized.