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.
Presidential spokesman Yasser Ali insisted the decree was nowhere near a declaration of martial law.
"It is merely a measure to extend legal cover for the armed forces while they are used to maintain security," he said.
The opposition has rejected the referendum, but has yet to call for a boycott or instead a 'no' vote at the polls.
"A decision on whether we call for a boycott of the referendum or campaign for a 'no' vote remains under discussion," said Hossam Moanis, a spokesman for the National Salvation Front grouping of opposition parties and groups. "For now, we reject the referendum as part of our rejection of the draft constitution."
The military last week sent out several tanks and armoured vehicles in the vicinity of the presidential palace in Cairo following protests there by tens of thousands of Morsi critics. It was the first high-profile deployment by the military since it handed power in June to Mr Morsi, Egypt's first freely elected president.