Deadly clashes were reported in provinces around the country, as police stations, government institutions and Coptic churches were attacked in apparent revenge attacks. Last night, the Ministry of Health put the toll at 421 killed, 3572 injured.
In a sign of how deeply the crackdown will affect Egypt's political transition, Mohamed ElBaradei, the Vice-President and Nobel laureate, resigned in protest over the crackdown.
Egypt's interim Government has imposed a month-long state of emergency and night-time curfews in Cairo, Alexandria and 12 other provinces.
Inside the Rabaa al-Adawiya Mosque, the building which lies at the heart of the east Cairo encampment, crying babies clung to their mothers as gunfire raged around them after the start of the operation.
In the centre of the prayer hall, laid out on the carpet among hundreds of women and toddlers in the stifling heat, 10 bodies had been placed side by side inside a cordon.
A little girl of about 7 or 8, wearing pink trousers and a T-shirt, made her way from one side of the mosque to the other by tottering between the heads of the corpses.
"The police and the army don't understand any language except force," said Khalid Mohsen, a 50-year-old engineer who was trapped inside the siege. "They want to kill anybody who has an opposing view."
Given the sheer level of firepower unleashed on protesters, it is a view which many Islamists may find hard to argue with. Security forces surrounded the site and a separate encampment in the west of the city.
Heavy semi-automatic bursts of gunfire echoed around the nearby suburbs throughout the day. If there was any let up, it was brief. For about 10 hours, the supporters of Mohammed Morsi were subjected to a near-continuous barrage of live fire.
At the nearby hospital, staff draped the windows with blinds as a precaution against sniper rounds.
One doctor at the hospital, who gave his name only as Ahmed, said that even the Israeli invasion of Gaza in 2008 had not been as bad.
"I was working there as a medic during that battle," he told the Independent. "The Jews were much more humane that what is happening today. Even in war, the rules are more respectable than this. In 12 days of fighting in Gaza, there were less dead than in six hours here."
Amid the dizzying chaos of the massacre - the third which has been perpetrated against Egypt's Islamists in a little over a month - reliable casualty figures were difficult to come by.
Dr Hisham Ibrahim, the head of the Rabaa al-Adawiya field clinic, said that several hundred had been killed.
Whatever the final tally, the constant stream of bullet-riddled, disfigured protesters meant it was impossible to store the corpses properly. Inside a room which during the previous two massacres has been used as a morgue, 42 bodies were crammed up against each other on the floor.
As the carnage unfolded and more protesters were killed, other areas were appropriated to house the dead.
At the main morgue beside the field clinic, the mother of one victim, 16-year-old Malik Safwat, struggled to reach him through the tightly-packed rows of corpses. She eventually found him, tearfully shaking his left knee from side to side as if to try and wake him up. His sister had also arrived. "My darling," she said in a trembling voice. "Why, my darling?"
Late in the day, the security services gained access to the hospital and were clearing everybody out into the surrounding streets. Thousands of people began filing out of the camp, as police bulldozers moved in to destroy the remaining tents.
- Independent
