Thousands of people have gathered in Cairo’s Nasr City to hold the Muslim prayers for the dead, to bid farewell to the victims of Monday clashes.

On Monday, at least 51 people, mostly supporters of deposed Egyptian President Mohamed Morsi, were killed in clashes with the security forces outside the Republican Guard barracks in Cairo.

The development came after the army opened fire on supporters of the Muslim Brotherhood in Cairo, prompting the Brotherhood to call for an uprising against the military.

The army says it confronted a group of armed men who were trying to enter the building. An army spokesman also noted that a soldier and a police officer have been shot dead in the attack.

The Brotherhood leaders and field doctors, however, reject the claim, saying that troops tried to break a sit-in by supporters of ousted president, Mohamed Morsi, while they were preparing for prayers.

Egypt’s Muslim Brotherhood has called for a nationwide protest on Tuesday, a day after the deadly incident.

On Monday, United Nations Secretary-General Ban Ki-moon condemned what was described by Morsi's supporters as a "massacre" and ordered an independent probe into the incident.

The Egyptian interim president also expressed “deep regret” for the victims of Monday violence in a TV statement, saying it has forged a judicial panel to investigate the incident.

Since last week, Egypt has been the scene of rival rallies and clashes between thousands of supporters and opponents of the ousted president.

Morsi was unseated on July 3, and the Chief Justice of Egypt’s Supreme Constitutional Court, Adly Mansour, was sworn in as interim president of Egypt on July 4.

Army officials said Morsi was being held “preventively” by the military and might face formal charges over accusations made by his opponents.