Egypt's chief justice Adly Mansour was sworn in as interim president on Thursday after the army ousted and detained Mohamed Morsi in an abrupt end to the Islamist's first year in office.
A senior military officer said the army was "preventively" holding Morsi. Security forces also rounded up Morsi's top aides and members of the Muslim Brotherhood movement to which he belongs.
Morsi's government unravelled on Wednesday after the army gave him a 48-hour ultimatum in the wake of massive demonstrations since June 30 against his turbulent rule.
Mansour, 67, took the oath of office at a ceremony at the Supreme Constitutional Court. Armed forces chief general Abdel Fattah al-Sisi announced Morsi's overthrow on state television late Wednesday, citing his inability to end a deepening political crisis.
Mansour would serve until new elections, according to the army's plan.