Ousted Egyptian President Mohammed Morsi is being held over allegations of links with Palestinian militants Hamas and plotting attacks on jails in the 2011 uprising, it has been announced.
He is to be questioned for an initial 15-day period, a judiciary order said.
The Islamist leader has been held as an undisclosed location since his removal by the military on 3 July.
The announcement comes as supporters and opponents of the deposed president prepare to stage mass rallies in Cairo.
The order issued on Friday said the deposed president was suspected of conspiring with Hamas, which rules the Gaza Strip and has strong links with Mr Morsi's Muslim Brotherhood, during the uprising against former President Hosni Mubarak.