At least 70 people have been killed after security forces attacked a protest by supporters of deposed President Mohamed Morsi in Cairo, according to the Muslim Brotherhood.
Doctors at a field hospital said another 1,000 people had been wounded in clashes on the road to Cairo's international airport.
"They are not shooting to wound, they are shooting to kill," Muslim Brotherhood spokesman Gehad El-Haddad said, adding that demonstrators had been hit by gunshot in the head and chest.
He said the shooting started just before pre-dawn prayers at a round-the-clock sit-in staged by Morsi supporters at Rabaa al-Adawiya in east Cairo.
Protesters set off flares in a mass protest in support of the army in Cairo Activists rushed blood-spattered casualties to the makeshift hospital, some carried on planks or blankets. Many had fatal head wounds.
A Reuters reporter at the scene counted 36 bodies at an improvised morgue.
Amid claims that rooftop snipers had opened fire on the vigil, Egypt's interior ministry blamed the Brotherhood for the violence.
An Apache helicopter flies over Tahrir Square during the protests The MENA state news agency, quoting an unnamed security official, reported that nine people had been killed in the violence and at least 200 wounded.
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."
Protesters stand on power lines near Cairo's presidential palace The violence broke out as rival rallies were held across Egypt for and against the overthrow of Mr 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.
A spokeswoman for the pro-Morsi camp said eight Brotherhood supporters had died in a clash near the Cairo vigil alone, and another said rooftop snipers had opened fire.
Lasers are pointed at an army helicopter near the palace At least 10 people have also been killed in Egypt's second city of Alexandria, where hundreds of people fought pitched battles, with birdshot fired and men on rooftops throwing stones at crowds below.
Several of those killed were stabbed, hospital officials said, and at least one was shot in the head.
The investigation into Mr Morsi over his 2011 escape from jail has signalled a clear escalation in the military's confrontation with the deposed leader and his Islamist movement.
MENA said Mr Morsi, who has been held at an undisclosed military facility since his overthrow, had been ordered detained for 15 days pending the inquiry.
Egypt's army-installed interior minister, Mohamed Ibrahim, said month-old Cairo vigils by Mr Morsi's supporters would be "brought to an end, soon and in a legal manner".
An army official said the military had given the party a Saturday deadline to end its resistance and join a military-set roadmap to fresh elections.
But the Brotherhood says it wants nothing to do with the army's transition plan and called its own crowds out for counter-demonstrations in a "day to remove the coup".
Mr Morsi has been in military detention at an undisclosed location since he was overthrown.
UN leader Ban Ki-moon has called on the military to free Mr Morsi and other Islamic Brotherhood leaders, said deputy UN spokesman Eduardo del Buey.

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