A doctor at a field hospital close to the protest at the Rabaa al-Adawiya mosque told the BBC that more than 1,000 people had also been injured.

The health ministry put the death toll at 20, with 177 wounded.

Both pro- and anti-Morsi supporters had been holding huge protests overnight in the capital.

Many thousands occupied Cairo's Tahrir Square in support of the army, which removed Mr Morsi from office on 3 July.

Army chief Gen Abdel Fattah al-Sisi had urged people to take to the streets to give the military a mandate for its intervention

Early on Saturday, Interior Minister Mohammed Ibrahim had vowed to end the sit-in at the mosque in the capital's Nasr City area.

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.

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 be a prepared campaign to clear the area around the mosque,

Tens of thousands of Morsi supporters remain camped in the protest zone.

It appears the violence began after some of the Morsi supporters tried to block a main road in the area overnight and security forces responded.

State news agency Mena quoted a security official as saying that live fire had not been used, only tear gas.

The official said security forces had been trying to stop fighting between rival sides and that eight security personnel had been injured.

But Muslim Brotherhood spokesman Gehad el-Haddad told Reuters news agency: "They are not shooting to wound, they are shooting to kill."

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.

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.

A field hospital in Cairo has been treating injured supporters of Mohammed Morsi (AFP)

Egypt's pro-Muslim Brotherhood TV station, Ahrar 25, quoted the coalition that supports Mr Morsi - the National Coalition for Supporting Legitimacy - as saying it held Gen Sisi responsible for the deaths at the mosque protest.

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.

European Union foreign policy chief Catherine Ashton said she "deeply deplored" the latest deaths in Egypt.

A spokeswoman said Baroness Ashton called "on all actors to refrain from violence and to respect the principles of peaceful protest".

UK Foreign Secretary William Hague said: "Now is the time for dialogue, not confrontation. It is the responsibility of leaders on all sides to take steps to reduce tensions."

Mr Morsi, the country's first democratically elected president, 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.

Mr Morsi and several Muslim Brotherhood leaders were freed during a breakout at a Cairo prison in January 2011.

Mr 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.

Rival rallies in Cairo

BBC Video here