More than 100 people are reported to have been killed in Cairo at a protest held by supporters of ousted Egyptian President Mohammed Morsi.
Clashes are still taking place around the Rabaa al-Adawiya mosque and there is blood on the streets, says the BBC's Quentin Sommerville at the scene.
A doctor at a field hospital close to the protest said that in addition to the dead, more than 1,000 were injured.
Both pro-and anti-Morsi supporters had been holding huge protests overnight.
Many thousands occupied Cairo's Tahrir Square in support of the army, which removed Morsi from office earlier this month.
Army chief Gen Abdel Fattah al-Sisi had urged people to take to the streets to give the military a mandate for its intervention.
It is not yet clear whether the clashes around the mosque represent a concerted effort by the security forces to clear the area.
Early on Saturday, Interior Minister Mohammed Ibrahim had vowed to end the sit-in at the mosque.
He said local residents had complained about the encampment and that the protest would be "brought to an end soon and in a legal manner".
The minister said the prosecutor would issue an order, but this has yet to happen.
Our correspondent says automatic gunfire can still be heard and the area is being hit by barrages of tear gas.
Muslim Brotherhood spokesman Gehad el-Haddad told Reuters news agency: "They are not shooting to wound, they are shooting to kill."
A senior Brotherhood politician, Saad el-Hosseini, told the agency that this was an attempt by security forces to clear the mosque area.
"I have been trying to make the youth withdraw for five hours. I can't. They are saying they have paid with their blood and they do not want to retreat," he said.
Our correspondent says the pro-Morsi supporters are furious about the role the military is taking, and in particular Gen Sisi, who they say is killing Egyptians.
There has also been violence in Egypt's second city of Alexandria, where at least 10 people have been killed in clashes between rival factions.
Since Morsi, the country's first democratically elected president, was ousted on 3 July, dozens of people have died in violent protests.
Morsi has now been formally accused of conspiring with the Palestinian militant group Hamas, which governs the Gaza Strip and has strong links with the Muslim Brotherhood.
He is alleged to have plotted attacks on jails in the 2011 uprising that overthrew President Hosni Mubarak.
Morsi and several Muslim Brotherhood leaders were freed during a breakout at a Cairo prison in January 2011.
Morsi is to be questioned for an initial 15-day period, a judicial order said.
The order issued on Friday was the first official statement on Morsi's legal status since he was overthrown and placed in custody at an undisclosed location.