Several Islamist presidential candidates announced they were suspending their campaigns -- some for 48 hours and some indefinitely -- in response to Wednesday's violence and what they said was the military-run government's unwillingness to stop it.
The Muslim Brotherhood demanded the ruling generals dismiss the government of Prime Minister Kamal Ganzouri and form a new one to run the country and oversee the upcoming presidential election.
Egypt's ruling Supreme Council of the Armed Forces denied responsibility for the violence in Cairo -- the deadliest in weeks, al-Masry al-Youm reported.
The council promised to hold a news conference Thursday to give its views about the clashes, which witnesses said began at dawn Wednesday when armed men attacked about 500 protesters demanding an end to army rule, the Middle East News Agency reported.
The witnesses said many of the mainly Islamist protesters outside the Defense Ministry had been stabbed and shot in the head and neck, McClatchy Newspapers reported.
The Islamist protesters were expressing support for ultraconservative Islamist preacher Hazem Salah abu Ismail, recently disqualified from the presidential race, the Los Angeles Times reported.
The military took several hours before it staged four tanks outside the perimeter of the fighting, but the security forces made no effort to stop the attacks, McClatchy said.
Protesters said the attack reminded them of an infamous assault on demonstrators demanding the ouster of then-President Hosni Mubarak in Cairo's Tahrir Square last year by thugs riding camels, which the security forces also failed to stop.
Who Wednesday's "thugs" were remained unclear Thursday.
Some protesters said they thought security forces had paid attackers or that the attackers had the tacit backing of the military.
Others suspected they could have been frustrated residents trying to get rid of protesters, McClatchy said.
Medical student Sayed Faithy, 20, who had joined the protest three days earlier, was among those who said the military had sanctioned the attack in an effort to delay the elections.
"Why before every election do massacres happen?" McClatchy quoted Faithy as asking. He then answered his own question: "They don't want to transfer power."
The first round of voting -- with Muslim Brotherhood party leader Mohammed Morsi, former Foreign Minister Amr Moussa and moderate Islamist Abdel-Moneim Abolfotoh the front-runners -- is set for May 23-24.