Screen shot of Egyptian security forces squaring off against protesters in Cairo, Egypt, July 8, 2013. Photo: Screenshot/CBS News.

Interim Egyptian President Adly Mansour urged calm Monday following the deaths of at least 50 Muslim Brotherhood protesters, during another violent day in Cairo.

In a statement read on state television,  Mansour’s office also expressed “deep sorrow” over the deaths Monday morning in Cairo.

The Muslim Brotherhood said its members were fired on as they staged a sit-in outside the Republican Guard building in the city.

A statement from the Islamist organization said “peaceful protesters were performing the Fajjr (dawn) prayers” when the army “fired tear gas and gunshots at them without any consideration for the sanctity of prayers or life.”

The Army said it was responding to a provocation by an “armed terrorist group,” the state-run Ahram online news site reported. Ahram online also reported that prosecutors have already begun an investigation into the bloody clashes.

Meanwhile, the Al Nour party, an ultraconservative Islamist party that has emerged as a political partner to the military, said that it was suspending its participation in efforts to form an interim government after Monday’s deaths, describing the incident as “a massacre.”

Regarding the formation of a government in Egypt, reports pointed to Ziad Bahaa El-Din, a lawyer and member of the Egyptian Social Democratic Party, as getting the nod to be the country’s interim prime minister as early as this Wednesday.

Mohamed ElBaradei, the former UN nuclear agency chief, who was expected to be appointed to the post before a key Islamist party rejected the proposal, would be appointed interim vice president under this new plan.