Vowing to stand their ground despite violent crackdown on their supporters by armed forces, Brotherhood leaders addressed protesters overnight, saying they would not back down from their demands including reinstatement of Morsi.
61-year-old Morsi, Egypt's first democratically elected president, was toppled by the military on July 3.
Morsi, who is facing criminal charges in many cases, was last seen in public on June 26 and has been detained along with senior aides of his Muslim Brotherhood party.
However, a Health Ministry official, Khaled El-Khatib, put the death toll from Friday and Saturday's clashes to 80.
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Seventy-two of the casualties fell during violence between police and pro-Morsi supporters on the fringes of a month-long sit-in held by the president's loyalists in northern Cairo's Rabaa al-Adawia mosque, Ahram Online said.
Eight people were killed in Egypt's Mediterranean city of Alexandria during deadly clashes between pro and anti- Morsi supporters.
The official put the tally of injured at 792 nationwide, including 411 in clashes near the pro-Morsi sit-in in Cairo's Nasr City neighborhood.
Yesterday's killings followed a day of rival rallies.
The Grand Imam of Al-Azhar mosque - the highest Sunni Muslim authority in Egypt - has called for an investigation, while the vice-president of the interim government, Mohamed ElBaradei, has condemned the excessive use of force.
The bloodshed has thrown Egypt into deeper turmoil weeks.
The violence has claimed the lives of dozens and wounded hundreds since Morsi's ouster from the presidency.