A deeply polarised Egypt is braced for bloodshed ahead of rival mass rallies called by the army and Islamists who back the ousted president Mohamed Morsi.
Army chief General Abdel Fattah al Sisi has summoned Egyptians to the streets in an intended turning point in its confrontation with followers of Mr Morsi, the elected leader the military removed on July 3.
Mr Sisi said he wanted Egyptians to give the military a "mandate" to take the necessary measures against "violence and terrorism".
It comes as the country's state news agency reported Mr Morsi had been detained over accusations he killed soldiers and conspired with the Palestinian group Hamas.
The claims were dismissed as "ridiculous" by his Muslim Brotherhood party, which has been camped out at street vigils calling for his reinstatement.
An army official said the military has 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 has 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.
Mr Ban also says he "urges all sides to act with maximum restraint" during the demonstrations.