Egyptian authorities have formally detained President Mohamed Mursi for 15 days over an array of accusations, including killing soldiers and conspiring with the Palestinian group Hamas.
Mr Mursi is accused of conspiring with Hamas, which rules the Gaza Strip and has strong links with his Muslim Brotherhood, to attack prisons and free fellow Islamists during the uprising against Hosni Mubarak in 2011.
The Mena news agency said Mr Mursi is accused of colluding with Hamas to storm police stations and jails, "setting fire to one prison and enabling inmates to flee, including himself, as well as premeditated killing of officers, soldiers and prisoners".
The statement provides legal cover for the continued detention at a time when the United Nations and Western powers are calling for the ousted president to be released or properly charged, the BBC reports.
The state news agency report on Friday came as millions of Egyptians were expected to take to the streets in mass rallies later in the day.
Army chief General Abdel Fattah al-Sisi has called Egyptians into the streets nationwide to give the military a "mandate" to confront weeks of violence unleashed by overthrow of the Islamist president on 3 July, Reuters reports.
The Muslim Brotherhood has called its own counter-demonstrations and has dismissed the accusations against Mr Mursi as "ridiculous".
"They are not taken seriously at all. We are continuing our protests on the streets. In fact we believe that more people will realise what this regime really represents - a return of the old state of Mubarak, with brute force," spokesperson Gehad El-Haddad said.
Both sides have warned of a decisive struggle for the future of the Arab world's most populous country.
It follows a month of clashes in which close to 200 people, mainly supporters of Mr Mursi, have died. The deposed ruler has been held by the military since his downfall.
Brotherhood supporters have been camped out in a Cairo square since 28 June, guarded by men with sticks behind barricades and sandbags. They fear a repeat of the killing of more than 50 Mursi supporters when security forces opened fire outside a Cairo barracks on 8 July.
The Brotherhood says it wants nothing to do with the army's transition plan. With Mr Mursi still in military detention at an undisclosed location, there is slim hope for compromise.
There is deepening alarm in the West over the course taken by the country of 84 million people, a pivotal nation between the Middle East and North Africa and recipient of some $US1.5 billion a year in mainly military aid from the United States.
Signalling its displeasure at recent events, Washington said this week it had delayed delivery of four F-16 fighter jets to Cairo and called on the Egyptian army on Thursday to exercise "maximum restraint and caution" during Friday's rallies.
United Nations Secretary General Ban Ki-moon has called on the military to release Mr Mursi and other detained Muslim Brotherhood leaders.