Egyptian President Muhammad Morsi has ordered the army to "protect government institutions" ahead of a December 15 constitutional referendum.
The decree issued late on December 9 gives army officers the authority to arrest civilians and orders them to cooperate with police in "maintaining public order."
It follows days of protests by the opposition to denounce the planned referendum and the new constitution which was drafted by Morsi's Islamist allies.
At least five people have been killed and hundreds injured in street clashes between supporters and opponents of the president.
In a bid to defuse tensions, Morsi annulled last month's decree which gave him nearly unrestricted powers. But he announced over the weekend that the referendum will go ahead as planned.
The opposition says the new constitution fails to protect fundamental freedoms as well as the rights of women and minorities.
Sameh Ashour, a spokesman for the National Salvation Front -- Egypt's main opposition group, has suggested a possible boycott of the constitutional vote.
"The National Salvation Front announces its complete rejection of the referendum scheduled for December 15," he said. "We reject this referendum which will certainly lead to more division and sedition."
The opposition has called for more protests on December 11. The Muslim Brotherhood, Morsi's Islamist allies, have announced plans for a rival rally on the same day.
Last week, the army deployed tanks outside the presidential palace in Cairo following deadly clashes between supporters and opponents of the president.
Morsi's decree, giving the army police powers, has raised fears about a possible return to military rule in Egypt.
The military ruled Egypt between the ouster of former President Hosni Mubarak in mass street protests in February 2011 and the election of Morsi in June this year.
The current standoff is the most serious political crisis in the country since Mubarak's ouster.
The military so far has sought to remain neutral, urging both sides to hold dialogue.
But it also warned it "will not allow" the situation to deteriorate.
With reporting by AP, dpa, AFP, and the BBC