Screen grab from a video showing an armed pro-Morsi supporter. This video was broadcast by several Egyptian television stations.
Early Monday morning, clashes broke out between the army and supporters of the Muslim Brotherhood near the Republican Guards’ headquarters in Cairo. The army said it was responding to a “terrorist attack”, while the protesters, who support ousted president Mohammed Morsi, claim they were not armed.
Confusion over how the violence started
According to a statement published by the Muslim Brotherhood; at dawn, soldiers and policemen opened fire on a crowd of protesters that had gathered to pray near the Rabia Al-Adawiya mosque, next door to the Republican Guard headquarters in Nasr City. Protesters said they were shot at with live bullets and tear gas grenades; some claimed those shooting were "thugs” in plainclothes working for the army.
The army, meanwhile, said that the violence was started by “armed terrorists” who attacked the Republican Guard building at dawn, leading to one death and several serious injuries on the authorities’ side. There are no images of this alleged attack, nor of the weapons the army claims the alleged attackers used.
Protesters go hunting for proof
The death toll from Monday morning’s clashes comes to more than 50 people, the vast majority of which were Morsi supporters. The Muslim Brotherhood denounced this as a “massacre”. Most of the wounded were taken into the Rabia Al-Adawiya mosque, which was quickly transformed into a field hospital. Inside, activists tried to gather proof to show the army attacked them.
According to FRANCE 24’s correspondent in Cairo, Gallagher Fenwick, Morsi supporters showed him “empty cartridges that they said came from AK-47s, weapons that only the army would have access to, according to the protesters”. Here is a photo of one of the cartridges that protesters showed our correspondent.
FRANCE 24 showed this photograph to a ballistics expert. He told us that this is a 7.62x39mm calibre cartridge, which could have been shot from an assault weapon like an AK-47 or a light machine gun.
Internet users have also been carrying out their own investigations. On the Rogue Adventurer, a blogger links certain types of ammunition photographed at the scene with ammunition typically used by the Egyptian army.
Photo taken at the scene of the clashes by journalist Louisa Loveluck. The letters "ARE" stand for Arab Republic of Egypt. 
In videos, Muslim Brotherhood supporters are seen carrying batons and stones
Many of those on the Muslim Brotherhood’s side say that none of them were armed when the first shots rang out on Monday morning. However, a doctor interviewed by The New Yorker said he witnessed firing from both sides while it was still dark outside. During the next few hours, @Mirna_elhelbawi, an Egyptian woman present near the scene of the clashes, wrote in a series of Tweets (translated here into English) that she also saw shots fired from the pro-Morsi side.
The first videos that surfaced of the clashes showed pro-Morsi protesters mainly armed with batons and stones. But later in the day, Egyptian television stations ran footage showing shots coming from the protesters’ side. These videos were used during an army press conference on Monday afternoon to justify its intervention.
Screen capture from a video broadcast Monday by Egypt's main official television station. At 1'02, a protester can be seen shooting in the soldiers' direction. 
In another video - this one filmed on Sunday - a pro-Morsi protester was filmed with a gun in his hand:
This video was filmed on Al-Taiaran Avenue, once again in Nasr City, and once again during clashes between pro-Morsi protesters and the army. It is difficult to determine exactly what kind of gun the man is holding. Some Internet users believe it to be a starting pistol or a BB gun. However, it could also be a handmade single-shot pistol.
A soldier filmed shooting in the protesters’ direction
Meanwhile, several videos showing soldiers shooting towards the pro-Morsi protesters have been widely shared on social networks. In the one below, a soldier up on a rooftop shoots in the direction of a street filled with protesters, some of whom are trying to evacuate wounded men. His weapon appears to be an AK-47.
The Freedom and Justice Party (FJP), which is the Muslim Brotherhood’s political party, has called for an “uprising” and urged the international community to prevent “a new Syria in the Arab world.” The FPJ’s headquarters in Cairo were shut down on Monday morning after the discovery of “flammable liquids, knives, and weapons”, according to the authorities.