Egypt's army has unleashed a deadly wave of attacks on Islamist protesters, killing up to 120 since Saturday.
The Cairo bloodshed was the worst since Morsi's ouster in a military-led coup on July 3, prompting domestic and international condemnation, as protesters accused security forces of using live ammunition.
“They had orders to shoot to kill,” said Gehad el-Haddad, a Brotherhood spokesman. The message, he said, was, “This is the new regime.”
US Secretary of State John Kerry, whose country contributes hundreds of millions of dollars in military and economic assistance to Egypt, expressed Washington's "deep concern" about the bloodshed.
US calls for Egypt to respect right to protest
In a statement, Kerry called on the authorities to "respect the right of peaceful assembly and freedom of expression".
Ahmed Aref, a spokesman for Morsi's Muslim Brotherhood, said 66 people were killed at Cairo's Rabaa al-Adawiya mosque, where supporters of the ousted president have been camped demanding his reinstatement.
An AFP correspondent counted 37 bodies in an Islamist-run field hospital at the mosque, and the emergency services said other hospitals received an additional 29 corpses