Egypt's Muslim Brotherhood says 70 people have now been killed during a peaceful demonstration for the country's ousted president.

It's accusing security forces of opening fire in Cairo, just before pre-dawn prayers.

"They are not shooting to wound, they are shooting to kill," said Muslim Brotherhood spokesman Gehad El-Haddad said.

The violence broke as rival rallies were held across Egypt for and against the overthrow of former President Mohammed Morsi, who is under investigation for murder.

Army actions

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.

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.

State news agency 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".

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.