Scores are dead after forces for the army-backed interim government began a crackdown on two mass demonstrations by supporters of the ousted president Mohamed Morsi.
World powers have condemned the show of force by Egypt's security forces.
But the interim government claims its security forces attempted to peacefully disperse protestors, and were fired upon.
For weeks, the interim government has been threatening Mr Morsi's supporters with imminent action to clear their camps in Cairo's Rabaa al-Adawiya and Nahda squares, which have brought parts of the capital to a standstill.
Then shortly after dawn on Wednesday local time, the security forces moved in.
Abdul Rahman Gamal is a doctor working in a makeshift field hospital near Cairo.
He says when the protest march came from Noor mosque in Abbasiya, security forces began firing live bullets at protestors.
He says medical professionals set up a triage in a nearby mosque, and have been treating people suffering from tear gas inhalation. And he says doctors have also treated about 20 cases of people shot by live fire, in the head, chest and legs.
Egyptian Interior Minister, Mohammed Ibrahim, says public warnings were issued, and people were told to evacuate before the military moved in to break up the crowds.
The government says hundreds of Morsi supporters were given safe passage out of the protest areas, but thousands stayed on.
Mr Ibrahim says instructions were issued to security forces that only tear gas was to be used to disperse the crowds, but he says when the soldiers arrived at the protest camps they were fired upon.
"We were surprised by a large number of protestors setting up curbs as barracks and taking certain positions, starting to fire live ammunition and cartridge heavily and intensely in the direction of the security forces who continued to maintain self-restraint."
But Mohammad Soudan, from the Muslim Brotherhood's political arm, the Freedom and Justice Party, has told Aljazeera the security forces are responsible for a massacre of the scale Egypt hasn't seen before.
"Just to kill at least 2600 people in a few hours. That's never happened before even in the war of 1973 against the Israeli troops. They've never been killed in such numbers."
The interim government says in addition to the casualties, 1,400 people have been injured, while the Muslim Brotherhood reports that the number of killed and wounded are in the thousands.
Britain's Sky News says one of its veteran cameramen has been shot and killed.
The West, notably the United States which gives the Egyptian military about 1-point-3 billion dollars each year, has been alarmed by the recent violence.
White House spokesman Josh Earnest.
"There's no question that the violence that we saw overnight is a step in the wrong direction. It is an indication that they're not currently following through on their promise to transition back to a democratically elected civilian government, that they're not committed to an inclusive process. It's time for them to get back on a path of respecting the basic human rights of their people."
Europe's leading powers along with Iran, Qatar and Turkey have also denounced the use of force.
In response, Egypt's vice president Mohamed El-Baradei has resigned.
Mr El-Baradei says his conscience was troubled over the loss of life, particularly as he says he believes it could have been avoided.
Egypt's army-installed authorities have declared a month-long state of emergency, and Cairo and other provinces have been put under a curfew between 7:00 pm and 6:00 am.
Meanwhile, Mr Morsi's Muslim Brotherhood is pushing for more protests.
It's using social networking websites to urge Egyptians to take to the streets to denounce what it's calling a "massacre".
