Egypt has been warned to pull "back from the brink" after security forces killed dozens of supporters of deposed President Mohamed Morsi during mass protests.
Thousands of Brotherhood supporters remain hunkered down in a vigil at a Cairo mosque, vowing to stand their ground despite threats by authorities to disperse them.
The Muslim Brotherhood says at least 120 people were killed after security forces attacked a protest by supporters of Mr Morsi in Cairo.
Doctors at a field hospital said another 1,000 people were wounded in clashes on the road to Cairo's international airport, while the Muslim Brotherhood was claiming that a total of 4,500 had been wounded.
The violence in Egypt has prompted condemnation from the international community, with the US Secretary of State John Kerry saying Washington had "deep concern" about the unrest.
Mr Kerry said Egyptian authorities had "a moral and legal obligation to respect the right of peaceful assembly and freedom of expression."
Supporters of Mr Morsi gather in Rabaa Adawiya square in Cairo
"This is a pivotal moment for Egypt. The United States ... calls on all of Egypt's leaders across the political spectrum to act immediately to help their country take a step back from the brink," he said in a statement.
The bloodshed follows huge rival rallies across the country and has plunged the nation deeper into turmoil following two turbulent years of transition to democracy.
Egypt's Health Ministry said 65 people had died. The Brotherhood said another 61 were on life support after what it described as a ferocious dawn assault by men in helmets and black police fatigues.
The ambulance service put the death toll at 72.
The Turkish Prime Minister, Recep Tayyip Erdogan, condemned the violence and said: "In Egypt, democracy was massacred, national aspirations were massacred, and now the nation is being massacred."
Pro-Morsi protesters chant slogans and wave flags in Egypt's capital
UK Foreign Secretary William Hague said in a statement that he was "deeply concerned" by the violence.
He said: "I am deeply concerned by recent events in Egypt, and condemn the use of force against protesters which has led to the loss of lives."
Bodies wrapped in white sheets have been laid on the floor of a Brotherhood morgue, their names scrawled on the shrouds.
In the early hours of Sunday, the state-run Al-Ahram news website reported fresh confrontations in the western Helwan district of Cairo between what it described as marching Brotherhood supporters and angry residents.
The report said several cars were destroyed and gunshots heard, but there was no information on casualties.
Over 200 people have died in violence since army chief General Abdel Fattah al Sisi deposed Mr Morsi on the back of huge popular protests against his rule.
This is so sad, a country I have LOVED for as long as I can remember truly on the brink of going under, I think we ALL need to look at this and understand people power is now showing itself in many countries. It started in the Middle East/North Africa, as the bible said it would (Strange how nobody is picking up on this huh?) and it is spreading. People want the freedom the people of the USA used to enjoy and the people in my Country still do, for now anyway. We here in the UK take to the streets to make a point, next day, we all go back to life. In the Middle East right now and North African, we could be seeing the 1st parting shots of a World sickened by Greed and Power hungry men. I think this could get REAL bad.
More love, Less Hate.