Officials in Egypt continue to add to the grim body count from yesterday's military assault on civilian protesters that may have been the single bloodiest day of the entire Arab Spring. The Egyptian Health Ministry puts the "official" death toll from Wednesday's attacks on Muslim Brotherhood protesters at 525 (update: the official death toll is now 638, and still expected to rise) but even that may not be a complete count of the carnage. The total has already been updated several times this morning, and The New York Times Cairo bureau chief David Kirkpatrick reports that another 250 dead bodies found in a Cairo mosque may not be included in that official figure. The total number of deaths recorded during the entire three weeks of the 2011 revolution toppled former President Hosni Mubarak in 2011 was 846.

Witnesses to yesterday's attacks described horrifying levels of violence as military and police forces gunned down mostly unarmed protesters. (One protestor told reporter Bel Trew of Foreign Policy, "They struck us down like animals... I can't tell you the amount of people who died in front of me.") Some were burned alive in their tents, while others were hit with tear gas canisters, bird shot, and the armored vehicles police used to clear out the sit-in camps that been growing for several weeks. Local mosques became makeshift hospitals and then morgues as bodies were lined up on the floor waiting to be identified, counted, and buried. At least four members of the media were killed trying to report from the scene.

It was the third such incident of mass killings since the army deposed President Mohammed Morsi at the beginning of July, but this was by far the most brutal. Police had originally suggested that they would cut off power and water to the camp in order to drive demonstrators out peacefully, which only made the overwhelming show of force all the more shocking. Reuters even reports that General Abdel Fattah al-Sisi, who leads the Egyptian army and is the de facto ruler of the country at the moment, ignored multiple warnings from Western diplomats to avoid a violent response to the protests. Despite that attempted pressure, the United States is now taking heat for failing both the prevent the massacre and for its half-hearted response, as the White House still refuses to fully withdraw military aid or even refer to the overthrow of Morsi as a coup.

For now, the Muslim Brotherhood remains resolute, calling for a new round of protest marches today, even though much of the nation remains under a curfew as part of an emergency law declared yesterday. A similar law was enacted in 1981 following the assassination of Anwar Sadat, and lasted for 30 years, until Mubarak was overthrown.

For more powerful photos from yesterday's events, including some that may be disturbing, both Al Jazeera and The Atlantic have collections of the best images.