According to friends and relatives, the moment of his own death was captured as the grainy film culminates.
News of Mr Assem's death filtered through after his bloodied camera and mobile phone were found at the site of the makeshift camp.
"At around 6am, a man came into the media centre with a camera covered in blood and told us that one of our colleagues had been injured," said Ahmed Abu Zeid, the culture editor of Assem's newspaper, who was working from a facility set up next to the Rabaa al-Adawiya mosque, about a mile away.
"Around an hour later, I received news that Ahmed had been shot by a sniper in the forehead while filming or taking pictures on top of the buildings around the incident.
"Ahmed's camera was the only one which filmed the entire incident from the first moment.
"He had started filming from the beginning of the prayers so he captured the very beginnings and in the video, you can see tens of victims. Ahmed's camera will remain a piece of evidence in the violations that have been committed."
Like much else about the deadly clash, the exact circumstances of the shooting are hard to prove. However, other witnesses to whom the Daily Telegraph spoke have described snipers being stationed on buildings overlooking the site, which is in an area dominated by military installations.
Excerpts of a 20-minute video said to have been recorded by Assem as the horror unfolded in front of him were shown at a Muslim Brotherhood press conference and are now being touted as evidence of a massacre on the streets of Egypt's capital.
The other video, which purports to show the final seconds before Assem was shot, have now been put on to his Facebook page, although the provenance of it could not be independently verified.
What is certain, friends say, is that Assem has left a vivid testimony of events whose origins have been hotly disputed. Mr Morsi's supporters say they were fired on from behind without provocation while they were praying. The army insists that security forces fired only after protesters attempted to storm the Republican Guard facility.
There have also been suggestions that the original firing may have come from agents provocateurs, triggering a wave of violence.
Whatever the truth, the Muslim Brotherhood says Assem's last film bears out its version of events and says it plans to use it as evidence.
However, Assem's brother, Eslam, said the footage's last seconds showed a soldier shooting demonstrators from a roof. The soldier then turned his gun towards Assem and the film suddenly went dead, he added.
Colleagues described Assem, a graduate of Cairo University's communications department, as a dedicated professional who had amassed an archive of 10,000 photographs since starting his career as a photographer three years ago.
His work for Al-Horia Wa Al-Adala - the official newspaper of the Freedom and Justice Party, the Muslim Brotherhood's political wing - put him in the front line of Egypt's political turmoil. It had also put him at odds with his family, who were supporters of the late Egyptian nationalist leader Gamal Abdal Nasser.
As Mr Assem's friends and family mourned, Adly Mansour, Egypt's new interim President, unveiled a draft constitution to replace the one drafted by Islamists and suspended last week. A committee will be set up to make final improvements to the document before it is put to a referendum. Parliamentary elections will then follow within three months and a date for a presidential election will be set once the Parliament has convened.
