This appears to be action from the military backed security forces clearing out a 'sit in' of pro-Morsi and Muslim Brotherhood supporters.
From Al-Jazeera:
Security forces have moved in on two Cairo protest camps set up by supporters of Egypt's ousted president Mohamed Morsi, launching a crackdown that quickly turned into a bloodbath with dozens dead.
Conflicting reports have emerged over the number of people killed on Wednesday. However, Al Jazeera's correspondent counted 94 bodies in Rabaa al-Adawiya's makeshift hospital, while some members of Morsi's Muslim Brotherhood said the death toll was as high as 2,200, with about 10,000 injured.
Al Jazeera could not independently verify the Brotherhood's figure.
Haven't seen a response from the White House but other international leaders have chimed in from the EU and other Arab states.
Turkey's President Abdullah Gul branded the crackdown as "unacceptable".
Germany's foreign minister, Guido Westerwelle, urged supporters of Egypt's interim government, as well as supporters of Morsi to renounce violence.
Fun times.