Egypt's interim leader has expressed sorrow over the deaths of at least 42 people near a barracks in Cairo, urging restraint amid ongoing unrest, BBC News reported.
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 "terrorists" tried to storm the barracks.
Scores have died since unrest began at the end of last month.
The killings follow an incident in the same location last week, in which three people died and dozens were wounded as troops fired on crowds.
Morsi, an Islamist and Egypt's first freely elected leader, was ousted by the army last week after mass protests. His supporters - many of them members of his Muslim Brotherhood movement - have been staging a sit-in outside the barracks, where Morsi is believed to be held, accusing the army of mounting a coup.
In a statement read out on state TV, Mansour's office expressed "deep sorrow" over the deaths in the "painful incidents" on Monday morning.
He urged self-restraint to uphold national interests and the country's security, adding that a judicial committee would investigate the deaths and announce its results to the public.
He gave the army's version of events, describing the incident as an attempt to storm the Presidential Guard barracks.
Mansour urged protesters not to approach the military or "vital installations".
Earlier the Freedom and Justice Party (FJP), the Brotherhood's political wing - which took nearly half the seats in historic parliamentary elections held in late 2011 and early 2012 - called on Egyptians to stage an "uprising" against "those trying to steal their revolution with tanks".
It also urged the international community to intervene to "stop further massacres" and prevent Egypt becoming "a new Syria".
The hardline Salafist Nour party - which had supported Mr Morsi's removal - said it was withdrawing from talks to choose an interim prime minister, describing the shooting incident as a "massacre".
