Supporters and opponents of Egypt's deposed president predicted blood on the streets ahead of what could be a violent day of mass protests, summoned by the Muslim Brotherhood and the army in a struggle for the country's future, Al Jazeera reported.
Protests were first called for Friday by Army chief General Abdel Fattah el-Sisi, who urged Egyptians to show their support for the overthrow of Mohamed Morsi and strengthen a military "mandate" to stop "violence and terrorism".
The Muslim Brotherhood in turn called for its supporters to fill the streets and oppose the military coup.
Confrontation appeared inevitable following a month of clashes in which close to 200 people, mainly supporters of Morsi, have died.
The army threatened to "turn its guns" on those who use violence, while the Brotherhood warned of civil war.
"We will not initiate any move, but will definitely react harshly against any calls for violence or black terrorism from Brotherhood leaders or their supporters," an army official told the Reuters news agency.
On Thursday rival protests filled Cairo's main squares, with pro-Mosri supporters camped in Nasr City and anti-Morsi demonstrators in Tahrir Square.
"Tomorrow we will cleanse Egypt," said Mohammed Abdul Aziz, a spokesman for the Tamarod ("Rebel") youth movement that helped rally millions in anti-Morsi street protests before the army moved against him on july 3.
"There are men carrying guns on the street," he told Reuters. "We will not let extremists ruin our revolution."
The rallies on Friday were expected to peak after the evening prayer marking the end of the day's Ramadan fast. Witnesses said army helicopters had dropped flyers at the Brotherhood vigil calling on people to refrain from violence.
The Interior Ministry said it would undertake "unprecedented measures to protect citizens and their property".
The country remains deeply split over the coup against Morsi on July 3. The Brotherhood accuses the army of ejecting a democratically elected leader in a long-planned coup, while its opponents say the army responded to the will of the people.
On Thursday, the US government said it did not intend to decide whether to class the removal of Morsi as a coup, in a move that avoids the cut-off of US.
The deputy secretary of state William Burns told members of Congress that the administration was not legally bound to draw any conclusion, and doing so would not be in the interests of the US.
"We believe that the continued provision of assistance to Egypt, consistent with our law, is important to our goal of advancing a responsible transition to democratic governance", the official said.