Despite at least 638 of its followers were killed in clashes with the Egyptian military, the Muslim Brotherhood is calling for a “day of anger” to follow Friday prayers to protest the deadly crackdown.

“Anti-coup rallies tomorrow will depart from all mosques of Cairo and head towards Ramisis (sic) Square after Jumaa prayer in ‘Friday of Anger’,” Muslim Brotherhood spokesman Gehad el-Haddad tweeted.

The antipathy between the two entities goes back to 26 October 1954 when the Muslim Brotherhood attempted to assassinate President Gamal Abdel Nasser (ensuring his rule through his death in 1970), and the military has kept a lid on the group until the Arab Spring in 2011.  But this week was the worst violence in Egypt in recent times.

The United Nations Security Council urged all parties to end the violence and exercise maximum restraint:

“The view of council members is that it is important to end violence in Egypt and that the parties exercise maximum restraint,” Argentina’s UN Ambassador Maria Cristina Perceval told reporters after the 15-member council met on the situation.  Argentina is the current holder of the Security Council’s rotating leadership.

“The members first of all expressed their sympathy to the victims and regretted the loss of lives,” said Perceval, “There was a common desire on the need to stop violence and to advance national reconciliation.”

Like US President Barack Obama earlier in the day, the Security Council took no concrete action.