Brotherhood spokesman Gehad El-Haddad said the shooting started shortly before pre-dawn morning prayers on the fringes of a round-the-clock sit-in being staged by backers of Mursi, who was toppled by the army more than three weeks ago.
"They are not shooting to wound, they are shooting to kill," Haddad said. The death toll might be much higher, he said.
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.
Al Jazeera's Egypt television station reported that 120 had been killed and some 4,500 injured in the early morning violence. A Reuters reporter at the scene counted 36 bodies at an improvised morgue.
