Anti-coup marchers are back on the streets of Egypt amid calls for an international probe into the deaths of 36 protesters while in police custody.

Crowds gathered on Monday evening in areas including Helwan in Cairo, Assuit, Zagazig and Gharbia. Troops and armoured vehicles were deployed to areas of Cairo, to guard government buildings.

The protests came hours after the Anti-Coup Alliance, which includes the Muslim Brotherhood, demanded a formal investigation into the deaths of 37 men being transported in a police van.

The alliance called the deaths an "horrific crime, in addition to other crimes committed by leaders of the 3rd of July coup".

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."

Al Jazeera's Mike Hanna, reporting from Cairo, 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.

"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. The 15-day order will begin at the end of a 30-day order issued on Thursday related to allegations of collusion with foreign groups and spying.

Morsi has been held by the military since his overthrow on July 3.