The Muslim Brotherhood said around 120 of its supporters were killed.
At least 60 people, largely supporters of the displaced Egyptian President, Mohammed Morsi were killed when protesters clashed with security forces in Cairo.
Security forces said they acted in self defence while the Muslim Brotherhood said its supporters were attacked by the security officials.
Egypt's health ministry confirms the death of 60 people while the Muslim Brotherhood said at around 120 of its supporters have been killed. Doctors at a field hospital in Nasr City, where the pro-Morsi supporters have been gathering at the Rabaah al-Adawiyah Mosque for nearly a month, told Aljazeera the number was as high as 120.
Egyptian authorities have said they would investigate the violence.
The Muslim Brotherhood's spokesman, Gehad El-Haddad, said Saturday's shooting started shortly before pre-dawn prayers on the fringes of the vigil being staged by backers of Mr. Morsi.
"They are not shooting to wound, they are shooting to kill," Mr. Haddad said, referring to security forces.
The death toll of 120, if confirmed, would be the largest casualty since Mr. Morsi, Egypt's first democratically elected president, was toppled in a military coup following protests against his government by tens of thousands of Egyptians.
Earlier on July 8, 53 people, mostly supporters of Mr. Morsi, died when armed men opened fire close to a Republican Guard compound in the area.
There are fears that the killings may continue as a 48 hour deadline issued by the military to Mr. Morsi's supporters to end their protests ended.
Egypt's current interior minister, Mohammed Ibrahim, warned a long-running sit-in in Cairo by Mr. Morsi loyalists would be ended "in the framework of the law".
Tens of thousands of Mr. Morsi's supporters, led by the Muslim Brotherhood, have vowed not to stop their peaceful protests until he is released and reinstated.
Mr. Morsi, who has been detained since the July 3 coup, has now been officially accused of "premeditated murder of some prisoners, officers and soldiers" for a prison break in 2011 during the presidency of former Egyptian dictator, Hosni Mubarak.
Some of the current officials of the interim government in Egypt all served the Mubarak regime, amidst reports that Mr. Morsi may be transferred to the same jail as the former dictator.