Egyptian police say at least 35 detained protesters have been killed while attempting to escape from a prison convoy, but the Muslim Brotherhood has alleged that their supporters were killed in cold blood and called for an international inquiry into the incident.
There were conflicting reports of how Sunday's deaths occurred. The Egyptian Interior Ministry said the prisoners had taken an officer hostage and died after suffocating to death after police fired tear gas.
"Thirty-five of the prisoners died of suffocation and crowding after tear gas was used to stop their escape," the Ministry said.
The men, believed to be supporters of deposed President Mohamed Morsi and his Muslim Brotherhood organisation, were killed while being transferred to Abu Zaabal prison near Cairo in a convoy of about 600 detainees, sources told Al Jazeera.
Al Jazeera was told another version of events by a source who said the prisoners staged a fight and then took a police officer hostage when the van pulled over. Other police then fired into the van, killing those inside.
The Egyptian MENA news agency said that the van transporting the men pulled over and was attacked by armed gunmen. It reported that the prisoners took a police officer hostage in an attempt to escape but were killed in a shootout, not by tear gas as claimed by the government.
Al Jazeera's Bernard Smith, reporting from Cairo, quoted a source who said the men had been arrested at the end of a siege at Cairo's Fateh mosque on Saturday. More than 200 people were said to have been arrested there on various charges including "terrorism".
Call for probe
In a statement on Sunday, the Anti-Coup Alliance said it had "obtained evidence of the assassination of anti-coup detainees in a truck transferring them to Abu Zaabal prison.
"They were reportedly assassinated in their truck with live ammunition and tear gas fired from windows."
"The murder of 35 detained anti-coup protestors affirms the intentional violence aimed at opponents of the coup, and the cold-blooded killing of which they are targets," it said in a statement in English.\
Abu Zabaal was the scene of a mass breakout of prisoners in 2011 as police abandoned their posts during protests against the former president Hosni Mubarak.
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The group said it “puts full criminal responsibility on leaders of the 3rd of July coup, beginning with Abdel-Fattah el-Sisi, [Egypt's army chief], and Minister of Interior Muhammad Ibrahim Kamel."
In the statement, the Anti-Coup Alliance demanded "an international investigation into this horrific crime, in addition to other crimes committed by leaders of the 3rd of July coup".
Sisi deposed Morsi on 3 July, saying that the army could not ignore the millions of protesters who had been demanding the resignation of Egypt's first democratically elected president.
On Sunday, Anti-Coup protesters broke a military curfew to march through Cairo, as the latest violence added to the rising death toll in days of unrest.
On Saturday alone, clashes between Morsi supporters and police killed 79 people, according to a government tally released on Sunday and carried by MENA, raising the death toll for four days of unrest across the country to over 800 people killed.
About 70 police officers were killed in clashes with protesters or retaliatory attacks during the same period, according to the Interior Ministry.