Cairo - Fifty-one people, mostly loyalists of Egypt's ousted president, were killed on Monday outside army barracks where they believed Mohammed Morsi is detained, prompting an Islamist call for an uprising amid US appeals for restraint.
Morsi's Muslim Brotherhood, which has led demonstrations against Wednesday's overthrow of the Islamist leader, said its supporters were "massacred" by troops and police during dawn prayers in Cairo.
The violence has delayed the naming of a new prime minister, who was to be announced on Monday, as the loose opposition coalition that backed Morsi's ouster lost a key Islamist party.
The Brotherhood released the names of 42 people killed in the incident, as the interior ministry and military said said two policemen and a soldier were also killed.
The military blamed "terrorists", while witnesses, including Brotherhood supporters at the scene, said security forces fired only warning shots and tear gas, and that "thugs" in civilian clothes carried out the shootings.
The United States called on the Egyptian army to exercise "maximum restraint", while also condemning "explicit" Brotherhood calls to violence.
The Islamist movement's political arm, the Freedom and Justice Party, had called for "an uprising by the great people of Egypt against those trying to steal their revolution with tanks" because of Monday's killings.
It urged "the international community and international groups and all the free people of the world to intervene to stop further massacres... and prevent a new Syria in the Arab world".
By nightfall, the Islamists held marches in several cities across country, the official Mena news agency reported.
But White House spokesperson Jay Carney said an immediate cut-off in military aid to Egypt "would not be in our best interests", when asked whether Washington was reconsidering the more than $1bn it provides annually.
In response to the "massacre", the conservative Islamist Al-Nur party, which won almost a quarter of the votes in 2011-2012 parliamentary elections and had backed the army's overthrow of Morsi, said it was pulling out of talks on a new government.
Grand Imam Ahmed al-Tayyeb of the Cairo-based Al-Azhar, Sunni Islam's highest seat of learning, said he would "remain in seclusion" until the bloodletting ends "and those behind it take responsibility".
The army-appointed interim president, Adly Mansour, set up a judicial commission of inquiry into the killings.
The bloodshed happened outside the headquarters of the elite Republican Guard, which the Brotherhood accuses of betraying Morsi, Egypt's first freely elected president.
Islamists hurled stones at the security forces who responded with tear gas.
"Morsi supporters were praying while the police and army fired live rounds and tear gas at them," said the Brotherhood.
Emergency services chief Mohammed Sultan told the official Mena news agency at least 51 people were killed and 435 wounded.
Emotions ran wild as people searched for the names of missing loved ones on a list of the dead in hospital, where dozens of bodies were laid on the bloody floor of a makeshift morgue.
The army said "armed terrorists" tried to storm the base, killing one security officer and critically wounding six.
It later warned it would not allow anyone to threaten national security, reiterated a call for protesters to stay away from military installations and urged them to end their sit-ins.
A security official said prosecutors later ordered the closure of the FJP's Cairo headquarters after police discovered weapons they alleged would be used against Morsi opponents.
International condemnation of Monday's bloodshed poured in, with Germany expressing "shock" at the violence, Turkey calling it an attack on "humanity" and Brotherhood backer Qatar urging "self-restraint" and "unity".
The violence came just hours before caretaker president Mansour had been due to name his interim prime minister.
It followed another day of duelling demonstrations across the Arab world's most populous nation of 84 million in an escalating crisis some fear could be exploited by extremist groups.
"There is a danger that some of them may resort to terror. If the army does not guarantee security, what is happening may make us wish for our dictators," political analyst Hisham Kassem said.
Morsi's single year of turbulent rule was marked by accusations he failed the 2011 revolution that ousted autocratic president Hosni Mubarak by concentrating power in Islamist hands and letting the economy nosedive.
The military, which overthrew Morsi after millions took to the streets on June 30 demanding that he resign, has come under mounting international pressure to swiftly install a civilian administration to oversee a rapid return to elected government.
Before the latest violence, Mansour aides had said he was leaning towards centre-left lawyer Ziad Bahaa Eldin as premier with ElBaradei as vice president, and that an announcement would be made on Monday.