Soldiers stand on an armoured personnel carrier positioned outside the state-run television station in Cairo - Reuters

Egypt's Muslim Brotherhood has warned it will fight back against the country's army after dozens of Mohamed Morsi's supporters were shot dead in Cairo.

At least 51 people were killed when troops opened fire on a pro-Morsi demonstration outside the Republican Guard headquarters where it was believed that the former president is being held.

The Brotherhood has called for an "uprising" against the army-installed interim administration over the killings - which, according to protesters, began during a peaceful sit-in protest.

One senior figure within the movement called the incident a massacre and demanded the reinstatement of Morsi.

Brotherhood spokesperson Gehad Haddad told Al Jazeera: "We have people hit in the head, we have bullets that exploded as they entered the body, cluttering organs and body parts.

"Every police force in the world understands how to disperse a sit-in. This is just a criminal activity targeting protesters."

The shooting took place just five days after Morsi was deposed by the Egyptian military and was likely to exacerbate the political crisis which has engulfed Cairo.

The army said that "an armed terrorist group" was behind the shooting, adding that nearly 200 armed demonstrators were detained.

The hardline Salafist Nour party, which campaigned for Morsi's overthrow, said it would withdraw from talks over the selection of an interim prime minister in response to the deaths.