Egypt's interim leader has expressed sorrow over the deaths of at least 51 people near a barracks in Cairo, urging restraint amid ongoing unrest.

Adly Mansour also said he had ordered an investigation into the deaths.

The Muslim Brotherhood says its members were fired on as they staged a sit-in for ousted President Mohammad Morsi, while the army said it had responded to an armed provocation.

The military has released footage which it claims shows pro-Morsi protesters shooting at soldiers stationed in front of the Republican Guard building.

Quentin Sommerville reports from Cairo.