Late-night violence at the site of a sit-in held by supporters of ousted Egyptian president Mohamed Morsi left at least 42 dead—nearly all protesters—and over 300 wounded, and led the right-wing Salafist party Al Nour to suspend its participation in the army's "roadmap" for transition.

In what the Muslim Brotherhood is calling a "massacre" and a "bloodbath," the Egyptian army and police apparently opened fire on protesters around 3:30 a.m. at the Republican Guard barracks where Morsi is thought to be held. "Every police force in the world understands how to disperse a sit-in," Brotherhood spokesman Gehad Haddad told Al Jazeera. "This is just a criminal activity targeting protesters."

The army says it was responding to a raid by a "terrorist group." Two officers were killed in the violence.

In response, the ultraconservative Al Nour party—a rival to the Muslim Brotherhood, and the group that had given the army's roadmap credibility among a broader grouping of Islamists—suspended its participation in talks. On Saturday, Nour, which has said it would prefer a prime minister with no party affiliation, had torpedoed diplomat Mohamed ElBaradei's chance at the premiership.

The National Salvation Front, the alliance of liberal, left and secular parties that organized the Tahrir Square demonstrations that led to the Army's removal of Morsi from power, has called for an "urgent and just" investigation.