Egypt's Anti-Coup Alliance has called for a formal investigation into the deaths of 36 detained protesters while being transported in a police van.
The alliance, which includes the Muslim Brotherhood, demanded on Monday "an international investigation into this horrific crime, in addition to other crimes committed by leaders of the 3rd of July coup".
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The men were killed on Sunday while being transferred on Sunday to Abu Zaabal prison near Cairo in a convoy of about 600 detainees, sources told Al Jazeera.
"The true account is not out yet, we have received three contradictory statements," the alliance said.
"We approached the chief attorney and requested a commission be formed and as we speak, the chief prosecutor has not taken any steps."
Anti-coup protesters were gathering on Monday evening for new rallies in areas including Zagazig and Gharbia.
The alliance had earlier said their supporters were killed in cold blood, but Egyptian security forces said the protesters suffocated when tear gas was used to stop them from escaping.
Al Jazeera's Mike Hanna, reporting from Cairo, cited a statement by alliance leaders saying that some of the bodies seen at the morgue bore marks of violence.
The statement blamed the military chief, General Abdel Fattah el-Sisi, and Mohammed Ibrahim, the Interior Minister, who is in charge of the police, for Sunday's incident.
"There have been conflicting reports as to what exactly happened," Al Jazeera's Hanna said.
"The initial reports from the Interior Ministry said that armed attackers had attempted to free the prisoners. This was then amended to the fact that prisoners being transferred from a police station to another prison had overcome one of the escorts, and they had to be attacked to free that particular escort.
|Anti-Coup Alliance calls for investigation into killings
"Then the version emerged that prisoners who were being transported from a police station or stations to another prison tried to escape, and tear gas was used which led to loss of life."
Earlier Al Jazeera's Bernard Smith reported from Cairo, citing a source, that the men had been arrested at the end of the recent siege at Cairo's Fateh Mosque on Saturday.
More than 200 people were said to have been arrested there on various charges including "terrorism".
On Monday, Egypt's public prosecutor ordered
the deposed president, Mohamed Morsi, to be detained for 15 days over allegations he participated in "Violent acts" last December, according to the state news agency MENA. Morsi has been held by the military since his overthrow on July 3, and faces other allegations of collusion with foreign groups and espionage.
Source: Al Jazeera and agencies