At least 42 people have been shot dead near a military barracks in Cairo, amid ongoing unrest over the removal of Egypt’s President Mohammed Morsi.
The Muslim Brotherhood says its members were staging a pro-Morsi sit-in at the barracks, where he is believed to be in detention, when they were fired on.
But the army said a “terrorist group” had tried to storm the barracks.
Mr Morsi, an Islamist and Egypt’s first freely elected leader, was ousted by the army last week after mass protests.
Scores of people have been killed since the unrest began at the end of last month.
Mr Morsi is believed to be detained at the Presidential Guard Club, in the eastern Nasr City district of the capital.
His supporters – many of them members of Mr Morsi’s Muslim Brotherhood movement – have been staging a sit-in there. They say the army has mounted a coup and are demanding his reinstatement.
The Freedom and Justice Party (FJP), the Brotherhood’s political wing – which took nearly half the seats in historic parliamentary elections held in late 2011 and early 2012 – called on Egyptians to stage an “uprising” in response to the incident, against “those trying to steal their revolution with tanks”.
It also urged the international community to intervene to “stop further massacres” and prevent Egypt becoming “a new Syria”.
After Monday morning’s violence, the hardline Salafist Nour party – which had supported Mr Morsi’s removal – said it was withdrawing from talks to choose an interim prime minister, describing the shooting incident as a “massacre”.
The Egyptian health ministry said at least 42 people had been killed, including an army officer, while the Brotherhood put the number of dead at 53, and said children were among the victims.
Some 300 people were reported wounded.
But there were conflicting reports over how the violence unfolded.
The Muslim Brotherhood said the army had raided its sit-in at about 04:00 (02:00 GMT) as protesters were performing dawn prayers.
“We found the army, around a thousand army troops surrounding us and firing guns, and on the other side of the street the riot police firing tear gas,” protester Mohamed Hassan told the Associated Press.
“They are pointing at our heads and chests. I saw with my own eyes 10 martyrs.”
In an emotional news briefing, members of the Brotherhood said military chief Gen Abdul Fattah al-Sisi was “an assassin and a butcher”.
But in a statement read on state media, the army blamed the shooting on “an armed terrorist group” that had tried to storm the barracks.
It said an army officer was among those killed and that a number of other soldiers were wounded, some critically.
The statement said some 200 people had been arrested and were found to have weapons, ammunition and petrol bombs.
Prosecutors have also ordered the closure of the FJP headquarters in Cairo, after police said weapons had been found inside.
The army later said two soldiers had been briefly kidnapped by Morsi supporters.
It said that in two separate instances men armed with guns and knives had forced the soldiers into vehicles and made them repeat pro-Morsi statements, filming one of them doing so.
The soldiers managed to escape, said the Mena news agency.
Mr Morsi was ousted on Wednesday by the military, which said it was responding to the demands of the people.
Protesters had been demanding that Mr Morsi step down, saying he had failed to tackle Egypt’s economic problems and was becoming increasingly authoritarian.
He was replaced on Thursday by Adly Mansour – the head of the Supreme Constitutional Court. Mr Mansour has pledged to hold elections soon, but as yet has given no date for them.
The army has insisted it does not want to remain in power.
On Sunday, tens of thousands of both supporters and opponents of Mr Morsi rallied in many Egyptian cities.
The BBC’s Jim Muir in Cairo says the latest violence has derailed efforts to find a political way out of Egypt’s crisis, leaving the country in a dire state.
The withdrawal of the ultra-conservative Nour party from the political transition talks will also set back efforts to appoint a new prime minister, our correspondent adds.
Party spokesman Nadder Bakkar said it had “decided to withdraw immediately from all negotiations in response to the massacre”.
Though the Islamist party had backed the army-led “roadmap” to new elections, it had blocked the appointment of two potential prime ministers because of concerns over the shape of a new constitution.
One of those rejected by Nour, leading opposition figure Mohamed ElBaradei, condemned the violence and called for an “immediate independent and transparent investigation”, saying Egypt was “in dire need of reconciliation”.