Scores of Islamists have died while reportedly attempting to escape during a prison transfer in the Egyptian capital Cairo, prompting the Muslim Brotherhood to accuse the security forces of murder.
Egypt's interior ministry said at least 36 people have been killed due to asphyxiation because of teargas and overcrowding.
The security forces earlier said they had thwarted an escape attempt by more than 600 prisoners, mostly supporters of ousted president Mohamed Morsi, from the Abu Zaabal prison located to the north of Cairo.
During the jailbreak, Egyptian officials say an armed group attacked a transfer vehicle containing many Brotherhood supporters, according to the official MENA.
Security forces shot dead some of the attackers, it has been reported. Government sources claimed that not all the dead were Brotherhood members.
However he anti-coup alliance led by the Brotherhood, which is fighting to reinstate Morsi, said it had "obtained evidence of the assassination of at least 38 anti-coup detainees in a truck transferring them to Abu Zaabal prison".
The group alleged that the prisoners were killed in their truck, with live ammunition and tear gas fired through the windows.
In 2011, during protests against the then-dictator Hosni Mubarak, the same prison witnessed a mass jailbreak during which scores of inmates fled.
"The murders show the violations and abuses that political detainees who oppose the July 3 coup get subjected to," said the Brotherhood.
The latest bout of violence has further increased the death toll in the escalating unrest between pro-Morsi demonstrators and government security forces, which began on 14 August.
The powerful Egyptian army, which removed Morsi from power, insists the military will not passively stand by and watch the protesters' violence.
"There is room for everyone in Egypt," said military chief General Abdel Fattah al-Sisi, addressing both military and police personnel.
Al-Sisi continued: "Whoever imagines violence will make the state and Egyptians kneel must reconsider; we will never be silent in the face of the destruction of the country."
He said the army-backed rulers had given many chances to end the crisis peacefully, and called on the followers of Morsi to participate in rebuilding the "democratic track and integrate in the political process and the future map instead of confrontations and destroying the Egyptian state".