The Muslim Brotherhood, the well-organized Islamist group which propelled Morsi into office and continues to demand his reinstatement, said in a statement that at least 60 people were killed and dozens injured in the attack on the larger of the two protest camps, at the Rabaah al-Adawiya mosque in the eastern Cairo district of Nasr City.
The state news agency said only that two members of the security forces were killed by gunfire.
CBS News' Alex Ortiz said police and army personnel had blocked every road into the area around the Rabaah camp, and pro-Morsi demonstrators were clashing with police outside the cordoned-off area, too. He said huge plumes of black smoke were rising over the camp and continuing automatic gunfire could be heard coming from the site.
Eventually Ortiz made it into a field hospital inside the cordoned-off area around the mosque, where he saw at least eight bodies of people killed from gunshot wounds to the head and chest. The youngest fatality seen by Ortiz appeared to be no more than 13 years of age.
Ortiz reported, however, that field hospital building is five stories high, and there were dead and wounded on every floor which he could not see. The stairs and floors were covered with blood and the sound of automatic gunfire from outside was nonstop.
Prominent Muslim Brotherhood leader Mohammed al-Biltagi told the Al Jazeera television network that as many as 300 people had been killed in the crackdown on the Rabaah camp, calling it a "crime against humanity."
"I urge all the Egyptians to take to the streets and protest against the violent break-up of the sit-ins," said al-Biltagi. "By Allah, I have seen toddlers being killed in front of their mothers."
There was no official word on casualties among the protesters in either camp. State television said only that two security force members were killed.
Sam Kiley, a veteran war correspondent for CBS News partner network Sky News, reported from the Rabaah camp that "live fire seems to be principle method being used to clear the area," adding that he had seen many dead, but that death tolls being reported were unlikely reliable because the "numbers are escalating constantly."
Video shot by army or police helicopters and broadcast by State TV showed what the government claimed were Muslim Brotherhood gunman firing at security forces from behind a sandbag barricade. The video could not be independently verified.
A former government aide who's been attending the Nasr City protest every day told CBS News that security forces took out a stolen State TV satellite truck first with live ammunition, then began firing volleys of tear gas into the camp.
The simultaneous actions by the Egyptian forces -- at the pro-Morsi encampment in Nasr City and at the site outside the main campus of Cairo University on the other side of the capital -- began around 7 a.m. local time. Helicopters hovered over the two sites.
Regional television networks were showing images of collapsed tents and burning tires at both sites, with ambulances on standby at the scene. They were also showing protesters being arrested and led away by the troops.
There were reports that the clashes had spread to other Egyptian cities, suggesting police and pro-Morsi protesters were facing off in Giza, Alexandria, Assiut and al-Minya.
At least 250 people have died in clashes in Egypt following Morsi's ouster in a military coup that followed days of mass protests by millions of Egyptians calling for his removal.