More than 100 people have been killed and 1,500 injured at a protest held by supporters of ousted Egyptian President Mohammed Morsi in Cairo, doctors say.
However the health ministry has put the death toll lower, at 38.
The army ousted Mr Morsi on 3 July. He has been formally accused of murder, relating to a 2011 jail outbreak, and of links to the militant group Hamas.
Both pro- and anti-Morsi demonstrators held huge protests overnight in the capital.
There were pools of blood all over the field hospital. Many of the injured were in bad shape. Some had parts of their head missing - taken out by bullets.
The battle raged last night and all morning. We heard automatic gunfire. Protesters formed a human shield around us as shotguns and CS gas were fired towards us. There was the sound of outgoing shots too. The air was thick with tear gas and people were vomiting.
There was a child, maybe 12, covered in blood. He was deathly white. They closed the doors at the hospital as they could not accept more wounded, but the ambulances were still arriving.
As we left, a child was dragged from a car by the Morsi supporters outside the mosque. He was given a very serious beating.
The anti-Morsi camp occupied Cairo's Tahrir Square in support of the army, after its chief, Gen Abdel Fattah al-Sisi, had urged people to demonstrate to provide a mandate for its intervention.
Meanwhile, tens of thousands of Morsi supporters continued their sit-in protest at the mosque in the Nasr City area.
On Saturday, Interior Minister Mohammed Ibrahim vowed to end the sit-in, saying local residents had complained about the encampment.
He said the protest would be "brought to an end soon, and in a legal manner" with an order from the prosecutor, although this has yet to happen.
The BBC's Jim Muir in Cairo says the latest violence is the most serious since the army's intervention to remove President Morsi, but this does not appear to have been a planned campaign to clear the area around the mosque.
'Shooting to kill'
It appears that clashes began after some of the Morsi supporters tried to block a main road in the area, and security forces responded.
The state news agency Mena quotes a security official as saying they had been trying to stop fighting between rival sides, and that eight security personnel had been injured.
The official added that live fire had not been used, only tear gas.
Muslim Brotherhood's spokesperson says the international community must step in
But our correspondent says medics at the hospital believed about 70% of the casualties were caused by live fire - with many of the victims hit in the chest or head by snipers firing from rooftops.
Ahmed Nashar, a Brotherhood spokesman witnessed what happened near the Nasr City mosque where demonstrators built a wall to protect themselves.
"When I arrived, bullets were whizzing past my ears," he told the BBC. "Today was just brutal - people were fired at, with live firearms."
Our correspondent says Morsi supporters are furious about the role the military is taking, and in particular the head of the army, General Abdel Fattah al-Sisi, whom they say is killing Egyptians.