Group has called on pro-Morsi supporters to stage "anti-coup rallies" after Friday prayers to protest deadly crackdown.
The announcement comes a day after hundreds of people were killed when security forces cleared two pro-Morsi protest camps, ending sit-ins that began after the army toppled Morsi on July 3.
"Anti-coup rallies tomorrow will depart from all mosques of Cairo and head towards Ramsis square after Jumaa prayer in 'Friday of Anger'," Muslim Brotherhood spokesman Gehad el-Haddad wrote on his Twitter account.
"Despite the pain and sorrow over the loss of our martyrs, the latest coup makers' crime has increased our determination to end them," the group said.
The Health Ministry spokesman on Thursday said that the death toll from nationwide violence in Egypt has climbed to 638, making it the bloodiest day since the Arab Spring in 2011 toppled longtime President Hosni Mubarak in a popular uprising.
Officials from the Muslim Brotherhood have said that the death toll was eight or nine times higher than the government figures.
Meanwhile, Egypt spent its first day under emergency rule and curfew on Thursday, with Cairo remaining tense and bracing for further unrest.
However, defiant Morsi supporters attacked the local government offices in Giza and set them ablaze. Television footage showed the headquarters in flames as men tried to douse the fire with hoses. Assailants used Molotov bombs to burn down the building.

There were also reports of clashes in Ma'adi, a neighbourhood in southern Cairo, between local residents and Morsi supporters, with witnesses saying both sides exchanged gunfire.
The army-backed interim government has defended the crackdown, saying authorities had no choice but to act.     
Defying criticism from major Western allies, Egypt's government warned it would turn its guns on anyone who attacked the police or public institutions after protesters torched a government building in Cairo.
Leftist coalition, the National Salvation Front, also called on Egyptians to protest against what it said was "obvious terrorism actions" conducted by the Muslim Brotherhood.
Warning that Egypt had entered a "more dangerous path", President Barack Obama cancelled US military exercises with the Arab country to protest the killing of hundreds of Egyptian protesters.
"We deplore violence against civilians. We support universal rights essential to human dignity, including the right to peaceful protest," he said.
The Egyptian presidency issued a statement saying Obama's remarks were not based on facts and would strengthen and encourage violent groups that were committing "terrorist acts."


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