"Strong Egypt Party demands an immediate, neutral investigation in this massacre," the party said in a press statement.

At least 51 pro-Morsi supporters were killed and hundreds wounded outside the Republican Guards headquarters early Monday morning.

But the army and the police insist that protesters attacked the vicinity of the Republican Guards headquarters and troops standing guard, killing two police officers and one army officer.

Interim President Adly Mansour has ordered the formation of a judicial committee to investigate the tragic events.

"We condemn in the strongest terms the killing of protesters outside the Republican Guards complex and holds the army responsible for protecting Egyptians," the Strong Egypt Party said.

"We also condemn this excessive use of force which claimed great number of fatalities."

The opposition National Salvation Front (NSF) earlier demanded an urgent and fair probe into what happened outside the army compound.

Al-Azhar Grand Imam Sheikh Ahmed El-Tayeb also urged Egyptian authorities to unveil the reality of what happened and keep the Egyptian people and the public opinion informed about the details of this "painful" incident.

Egypt Freedom party chairman Amr Hamzawi underlined the need to bring to justice the perpetrators of what he described as a "massacre."

The Salafist Nour party has decided to withdraw from the political process under the roadmap announced by the military last week in reaction to the killings.

"(We) will not deal with Adly Mansour, the head of the constitution court – as interim president," said party leader Younis Makhyoun.

Thousands of supporters have been camping outside the Republican Guards compound to protest the ouster of Morsi, Egypt's first democratically elected president.