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.
There were no signs of a beefed-up military presence outside the presidential palace, the site of fierce street clashes last week, or elsewhere in the capital.
Still, Mr Morsi's decision to lean on the military to safeguard the vote is widely seen as evidence of just how jittery the government is about the referendum on the draft constitution, which has been at the heart of days of protests by the opposition and Mr Morsi's Muslim Brotherhood backers.
The two sides clashed in Cairo last week, leaving at least six people dead and hundreds wounded in the worst violence of the crisis.
Both the opposition and Mr Morsi's supporters have called for mass rallies on Tuesday.
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.
Mr Morsi has rescinded decrees issued on November 22 granting him near absolute powers and placing him above any oversight, including by the courts. He has, however, insisted that the referendum will go ahead on schedule.
Judges have gone on strike to protest against Mr Morsi's perceived "assault" on the judiciary and have said they would not oversee the December 15 vote as is customary for judges in Egypt.
Judges of the nation's administrative courts announced they were conditionally lifting their boycott of the vote, but they said their supervision of the process was conditional on bringing an end to the siege of the Supreme Constitutional Court by Mr Morsi's supporters.
In exchange for their supervision, they also demanded assurances that authorities would crack down on vote canvassing outside polling stations and offer life insurance policies to the judges.
Mr Morsi's deputy, Mahmoud Mekki, has said the vote could be staggered over several days if there were not enough judges to oversee the referendum.
The court was widely expected to dissolve the panel that drafted the constitution in a session scheduled for December 2. The siege of the Nile-side building in Cairo's Maadi district began on December 1.
In a surprise move today, Mr Morsi has also rescinded a series of decrees issued on Sunday to raise taxes on a wide range of items and services, including alcohol, cigarettes, mobile phones, services offered by hotels and bank loans.