Seeking to break up a demonstration that was blocking a key road artery in Cairo, the Egyptian army apparently fired live ammunition into a crowd of thousands of pro-Morsi supporters, killing dozens and wounding many hundreds.

Exact numbers of dead and wounded are hard to come by. Al-Jazeera claims 120 have died with over 1000 wounded. This Reuters report tallies 57 dead with on scene reports saying that up to 80 were killed. The Muslim Brotherhood echoes the al-Jazeera number of 120 dead while claiming 4,500 wounded.

Whatever the number, the change in tactics by the police appears to have ratcheted up the crisis a couple of notches.

Reuters:

Men in helmets and black police fatigues fired on crowds gathered before dawn on the fringes of a round-the-clock sit-in near a mosque in northeast Cairo, Mursi's Muslim Brotherhood said.

"They are not shooting to wound, they are shooting to kill," said Brotherhood spokesman Gehad El-Haddad. "The bullet wounds are in the head and chest."

A Muslim Brotherhood website said 120 people had been killed and some 4,500 injured. A Reuters reporter counted 36 bodies at one morgue, while health officials said there were a further 21 corspes in two nearby hospitals.

Activists rushed blood-spattered casualties into a makeshift hospital, some were carried in on planks or blankets. One ashen teenager was laid out on the floor, a bullet hole in his head.

Egypt's Interior Minister Mohamed Ibrahim told reporters only 21 had died and denied police had opened fire, accusing the Brotherhood of exaggerating for political ends.

Ibrahim said local residents living close to the Rabaa al-Adawia mosque vigil had clashed with protesters in the early hours after they had blocked off a major road bridge. He said that police had used teargas to try to break up the fighting.

Well over 200 people have been killed in violence since the army toppled Mursi on July 3, following huge protests against his year in power. The army denies accusations it staged a coup, saying it intervened to prevent national chaos.

The Arab world's most populous state is battling economic woes and struggling with the transition to democracy two years after Hosni Mubarak was swept from power in the Arab Spring.

Hundreds of thousands of Egyptians had poured onto the streets on Friday in response to a call by army chief General Abdel Fattah al-Sisi for nationwide demonstrations to give him backing to confront the weeks-long wave of violence.

His appeal was seen as a challenge to the Brotherhood, which organized its own rallies on Friday calling for the return of Mursi, who has been held in an undisclosed location since his ousting and faces a raft of charges, including murder.

Brotherhood leaders appealed for calm on Saturday, but activists at the Rabaa al-Adawia mosque vigil voiced fury.

There were reports of snipers on nearby rooftops. Indeed, most of the dead suffered head and chest wounds from high powered rifles.

But the police are reporting that they were fired on first, with several officers suffering gun shot wounds. This would indicate that the Muslim Brotherhood knows exactly what it's doing as far as trying to gin up outrage against the military. There probably aren't a lot of Brotherhood armed cadres in those crowds - but there are probably enough of them to provoke a violent response from the army.

If this is a change in tactics by the army, there are going to be many more massacres like this one.