Egyptian lawyers called on Monday for an international investigation into the deaths in police custody of 37 supporters of Islamist President Mohamed Mursi.
A coroner's report said the men died from suffocation after police used teargas to stop a mass escape on Sunday while a group of more than 600 suspects were being transported to the Abu Zabal prison on the outskirts of Cairo.
Photos provided by the lawyers representing the detainees show dead bodies with charred faces and limbs and others covered in bruises which the lawyers said were signs of torture. Details of the incident remain unclear, they said.
"Neutral investigation committees must be formed, not Egyptian ones ... The real crisis is that the judiciary body is providing a veil to cover those who violate the law and sanction bloodshed," said Ahmed Abu Baraka, one of the lawyers representing the 600 detainees.
Security officials denied accusations of foul play during Sunday's incident.
At least 850 people have died since Wednesday in clashes between security forces and supporters of Mursi, who was overthrown by the army on July 3 following mass protests.
Egypt's army-backed government has accused Mursi's Muslim Brotherhood movement of seeking to destabilise Egypt.
Hundreds of Brotherhood supporters have been arrested in recent days in an effort to end weeks of protests. The detainees who were killed on Sunday were among those held after security forces stormed two pro-Mursi camps in Cairo on Wednesday.
Mustafa Azab, the spokesman for a committee of lawyers formed to defend the detainees, said they had complained to the International Criminal Court in the Hague about the killings.
"We have submitted to the International Criminal Court reports of all the massacres and we will submit more ... We will address the United Nations and all the entities that Egypt is a member of," he said.
"We have a huge problem in the loss of neutrality among state institutions," said Azab.
"They have the forensics, the police, the army, the informants and the intelligence so all the reports that the judiciary needs are in their hands," he said, adding that the lawyers had been denied any communication with the detainees.