Scores killed as rallies in Egypt turn violent
Dozens killed and more than 500 injured as hundreds of thousands come out for rival rallies across the country.
At least 90 people have been killed when security forces attacked a protest by supporters of deposed President Mohamed Morsi in Cairo, according to the Muslim Brotherhood.
"They are not shooting to wound, they are shooting to kill," said Muslim Brotherhood spokesman Gehad El-Haddad said, adding that demonstrators had been hit by gunshot in the head and chest.
The violence broke as rival rallies were held across Egypt for and against the overthrow of Morsi, who is under investigation for murder.
Hundreds of thousands of Egyptians heeded a call by army chief General Abdel Fattah al Sisi to take to the streets, while the Muslim Brotherhood mounted counter-demonstrations in Cairo.
In Alexandria, seven people were earlier reported killed and hundreds injured in clashes between supporters and opponents of Morsi.
The violence came on a night of protests called for on Wednesday by the army chief, General Abdel Fattah el-Sisi, to create a mandate against what he called "terrorism" by opponents of the coup against Morsi.
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".
However, it is not clear whether the latest clashes around the mosque represent a concerted effort by the security forces to clear the area.
Muslim Brotherhood spokesman Gehad el-Haddad told Reuters news agency: "They are not shooting to wound, they are shooting to kill."
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 Mr Morsi's legal status since he was overthrown and placed in custody at an undisclosed location.