Egyptian Islamists called for an uprising after saying dozens of its supporters were "massacred" Monday while demonstrating against last week's military coup, dashing the army's hopes of broad support for an interim civilian administration.
The Muslim Brotherhood movement, which has refused to accept the army's toppling of its champion Mohamed Morsi, who won Egypt's first freely contested presidential election, called for international intervention to prevent a Syria-style civil war.
The bloodshed outside the Cairo headquarters of the elite Republican Guard came just hours before the caretaker president installed by the army had been due to announce his choice of interim prime minister.
It had been delayed by strong opposition from the ultra-conservative Islamist Al-Nur party to president Adly Mansour's first choice, Nobel Peace laureate Mohamed ElBaradei, and the party announced it was pulling out of talks entirely in response to Monday's "massacre".
The Brotherhood said its activists were holding dawn prayers at their protest camp outside the Guard's headquarters when security forces opened fire, leaving at least 35 of them dead.
"Morsi supporters were praying while the police and army fired live rounds and tear gas at them. This led to around 35 dead and the figure is likely to rise," the Brotherhood said.
A senior medical official told AFP at least 42 people were killed and 322 wounded in the shooting.
The army said "armed terrorists" tried to storm the base, leaving one security officer dead and six critically wounded.
The Brotherhood's political arm, the Freedom and Justice Party, called for "an uprising by the great people of Egypt against those trying to steal their revolution with tanks".
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".
The violence overshadowed nationwide demonstrations held on Sunday in support of the army and threatened the already delayed announced of an interim government.
The military has come under mounting international pressure to swiftly install a civilian administration to oversee a rapid return to elected government.
The United States has specific legislation against military overthrows of elected governments that could imperil Washington's $1.3 billion a year in military aid.
Police barricades prevented journalists from accessing the area around the Republican Guard base, already the scene of deadly violence last Friday.
Demonstrators said troops had fired tear gas canisters and warning shots in a bid to disperse them but a group of men in civilian clothing had fired live rounds straight into the crowd.
"The Republican Guard fired tear gas but the thugs came from the side. We were the target," protester Mahmud al-Shilli told AFP.
The Al-Nur party, which won almost a quarter of votes in a 2011 parliamentary election and had given its support to the army's overthrow of Morsi, said it was pulling out of talks on a new government in response to the "massacre".
"We have decided to withdraw immediately from all negotiations in response to the massacre outside the Republican Guard" headquarters, Al-Nur spokesman Nadder Bakkar said on Twitter.
The party had already voiced strong opposition to Mansour's first choice, ElBaradei, an outspoken liberal opponent of the Brotherhood, delaying efforts to form an interim government.
Mansour aides had said before the deadly violence that he was leaning towards appointing centre-left lawyer Ziad Bahaa Eldin as prime minister with ElBaradei as vice president and that an announcement would be made later on Monday.
Hundreds of thousands of Morsi opponents took to the streets of Cairo on Sunday to demonstrate their support for his overthrow.
Wave after wave of military aircraft flew over the demonstrators, with one formation leaving behind long trails of smoke in black, white and red -- the colours of the Egyptian flag.
"We are on the street to show the world that it was a popular revolution and not a coup that overthrew" Morsi, said a beaming teacher who gave her name as Magda.
Many banners expressed anger with the United States for what they perceive as its support for Morsi, as well as American media coverage depicting his ouster as a coup.
"America shame on you! This is a revolution, not a coup!" read one.
President Barack Obama insisted the United States was "not aligned" with any political party or group in Egypt following Morsi's ouster.
"The future path of Egypt can only be determined by the Egyptian people," the White House quoted him as saying.
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.