A Mursi supporter mourns the death of a relative during clashes with security forces in Cairo on Saturday. (AP)

Cairo, July 27: The police and armed civilians opened fire today on protesters against Egypt’s new military government, witnesses said, killing scores of people as hopes faded that the Egyptian military would reach any political accommodation with the Muslim Brotherhood and its ousted President, Mohamed Mursi.

Clashes between the police and Mursi’s supporters erupted at about 11.30pm (local time) yesterday.

Witnesses said the police were trying to disperse protesters as they approached a central Cairo bridge, using tear gas at first, but then the officers, joined by armed civilians, fired birdshot and ultimately live ammunition to drive the protesters back.

By later this morning the bodies of at least 29 protesters were seen laid out on concrete floors in a makeshift morgue, while there were 20 more dead at a nearby hospital. The Muslim Brotherhood put the death count at 70 people overall.

‘'They are not shooting to wound, they are shooting to kill,'’ said Gehad El-Haddad, a spokesman for the Brotherhood.

The burst of violence came after a vast state-orchestrated display of military power yesterday, with army helicopters hovering low over a huge throng of flag-waving, pro-military demonstrators in Tahrir Square and soldiers deploying in armoured personnel carriers across the capital.

The crowds had turned out in Cairo and other Egyptian cities in response to a call by the defence minister, Gen. Abdul-Fattah el-Sisi, for mass demonstrations he said would give him a “mandate” to fight terrorism, a phrase widely understood to mean crackdowns on the Brotherhood.

The mass gathering was another blow to the Arab world’s most prominent Islamist group, which until recently was the major political force in government, having repeatedly won elections after the country’s uprising two years ago.

The Brotherhood and several western and Arab diplomats had called for the military, which has held Mursi incommunicado since his ouster three weeks ago, to release him as a good-will gesture, in hopes of brokering a compromise that would end the standoff between Islamists and the military. That now seems almost impossible, analysts say, with indications that the military is carrying out investigations geared towards a broader legal assault on the Brotherhood.

“This is a preparation for eliminating the Brotherhood,” said Emad Shahin, a political science professor at the American University in Cairo.

The Brotherhood responded defiantly yesterday, with pro-Mursi marches taking place along dozens of planned routes in Cairo and other cities.

The group has continued to demand Mursi’s reinstatement as a precondition for any negotiations.