More than 90 people have been confirmed killed after Egyptian security forces opened fire as they tried to clear two protest camps loyal to deposed president Mohamed Morsi in Cairo.
A month-long state of emergency has been declared as violence spread from the capital to other parts of the country including the Mediterranean city of Alexandria.
A curfew from 7pm to 6am has been declared in Cairo, according to reports, as well as several other provinces.
The health ministry put the number of dead in Cairo at 95, with hundreds more injured. But the Muslim Brotherhood claimed hundreds had been killed.
Reporter Sam Kiley, reporting from inside the Rabaa al Adawiya camp in the capital, said it was "under very heavy gunfire" and was a "massive military assault on largely unarmed civilians in very large numbers".
He said government forces were using machine guns, snipers, AK-47 and M16 rifles and were firing into the crowd.
Kiley added "There are machine gun rounds, and snipers on the roof, that are preventing people from getting any closer to the field hospital (in the camp). I haven't seen any evidence yet of any weapons on the side of the pro-Morsi camp. The camp is very full of women and children."
He said it was a scene of "extreme chaos and bloodshed" and "many hundreds of troops and interior ministry police and special forces are involved".
"The dead and dying are on the steps of an improvised field hospital. The scenes here are absolutely graphic. I have covered many wars and this is as severe a battlefield as I have witnessed, with the exception of scenes in Rwanda. There are dozens and dozens of people who have been shot in the head, neck and upper body."
Among those reported killed in the camp was Asmaa al Beltagui the 17-year-old daughter of senior Muslim Brotherhood leader Mohamed al Beltagui.
Sky News cameraman Mick Deane was shot & killed in Cairo
Meanwhile, Egyptian authorities have released video footage taken from a helicopter which it said showed gunmen in the camp firing at security forces.
The unrest spread beyond the capital, as pro-Morsi supporters clashed with police in the Nile Delta cities of Minya and Assiut, as police stations, government buildings and churches were attacked or set ablaze.
In Alexandria, tear gas canisters rained down on a pro-Morsi march in the Sharq neighbourhood, amid repeated bursts of automatic gunfire.
Residents, armed with clubs, came out of their homes and shops to help the police, detaining Morsi supporters and handing them over to officers at the Sharq police station.
Morsi supporters, carrying Egyptian flags and pictures of the deposed leader, then clashed with his opponents on a road carpeted with rocks.
Earlier, riot officers in Cairo backed by armoured vehicles and bulldozers also fired tear gas in the camps at the demonstrators who are demanding Mr Morsi be reinstated as the country's leader.
The interior ministry, which is in charge of police, warned its security forces would deal firmly with protesters acting "irresponsibly" and said it would guarantee safe passage to those who want to leave the two sites.
The larger is the Rabaa al Adawiya camp described as a 'mini town' in Nasr City, while the other is in Al Nahda Square outside the main campus of Cairo University in Giza.
The interior ministry later said security forces had "total control" over the smaller camp and police have managed to remove most of the tents in the square.
The Muslim Brotherhood that backs ousted Islamist president Mr Morsi claimed over 250 people had been killed and 5,000 hurt in the crackdown, which is almost certain to deepen political turmoil in Egypt.
It urged Egyptians to take to the streets in their thousands to denounce the "massacre".
"This is not an attempt to disperse, but a bloody attempt to crush all voices of opposition to the military coup," Brotherhood spokesman Gehad al Haddad said on Twitter.
The Rabaa al Adawiya protest camp, where several Brotherhood leaders are staying, "is calling on Egyptians to take to the streets to stop the massacre," Haddad said.
At least three members of the security forces were confirmed to have died in the crackdown, while the health ministry said nine protesters were killed and over 80 were injured.
The raids came after international efforts failed to mediate an end to a six-week political standoff between Morsi's supporters and the army-backed government which took power after he was ousted on July 3.
Regional television networks showed images of collapsed tents and burning tyres at both sites, as well as protesters being arrested and led away by troops.
A television feed by a pro-Morsi TV station showed thousands of protesters gathered at the centre of the Nasr City site, with many covering their faces to fend off the tear gas.
It said most of the protesters at the other camp fled to the nearby Orman botanical gardens and inside the sprawling university campus.
The Minister for Foreign Affairs Eamon Gilmore said today "I deplore the loss of life in Egypt, appeal for the authorities to show restraint and for all involved to refrain from violence".
Qatar, Turkey and Iran were among the other countries criticising the deadly crackdown.