The military has assumed responsibility for security and protecting state institutions in Egypt until the results of a constitutional referendum on December 15.
The army took up the task in line with a decree a day earlier from President Mohammed Morsi.
The presidential edict orders the military and police to jointly maintain security in the run-up to Saturday’s vote on the disputed charter that was hurriedly approved last month by a panel dominated by the president’s Islamist allies.
The decree, which also grants the military the right to arrest civilians, is seen as evidence of how jittery the government is about the referendum, which the main opposition parties have rejected.
The edict takes effect on the eve of mass rallies called by the opposition and Mr Morsi’s supporters.