Egypt's army threatened on Thursday to shoot those who use violence in a stark warning before what both sides expect will be a bloody street showdown between Islamists and opponents of deposed President Mohamed Morsi.
An army official said the military had set Morsi's Muslim Brotherhood an ultimatum, giving it until Saturday to sign up to a plan for political reconciliation which it has so far spurned.
The army has summoned Egyptians into the streets on Friday in an intended turning point in its confrontation with followers of Morsi, the elected leader the generals removed on July 3.
The Muslim Brotherhood, which has maintained a street vigil for a month with thousands of supporters demanding Morsi's reinstatement, has called its own crowds out for counter-demonstrations across Egypt in a "day to remove the coup."
Both sides have dramatically escalated rhetoric before Friday's demonstrations. The Brotherhood accused the army of pushing the nation towards civil war and committing a crime worse than destroying Islam's holiest site.
In a Facebook post, the army said it will not "turn its guns against its people, but it will turn them against black violence and terrorism which has no religion or nation".
A military official said the army had given the Brotherhood 48 hours from Thursday afternoon to join the political process. He did not say what would happen if it refuses.
Army chief General Abdel Fattah al-Sisi has called on Egyptians to take to the streets and give him a "mandate" to act against the violence that has convulsed Egypt since he shunted its first freely elected president from power.
The Brotherhood, which has won repeated elections since the fall of President Hosni Mubarak in 2011, accuses the authorities of stirring up the violence to justify their crackdown.
Sheikh Youssef al-Qaradawi, an influential Egyptian cleric based in Qatar, issued a religious edict broadcast on Al Jazeera television urging soldiers to disobey orders to kill.
"I call on officers and soldiers in the Egyptian army not to listen to what al-Sisi says, or anyone else. Do not kill anyone. Do not kill your brothers. It is forbidden," Qaradawi declared.
The main anti-Morsi youth protest group, which has backed the army, said it would go to the streets to "cleanse Egypt".
The West is increasingly alarmed at the course taken by Egypt, a strategic hinge between the Middle East and North Africa, since protests in 2011 brought down Mubarak and ended decades of autocratic rule in the most populous Arab state.
Signaling its displeasure, Washington has delayed delivery of four F-16 fighter jets to Cairo. On Thursday, the White House urged the army to exercise "maximum restraint and caution".
The United States has yet to decide whether to call the military's takeover a "coup", language that would require it to halt $1.5 billion it sends in annual aid, mostly for the army.
For weeks, the authorities have rounded up some Brotherhood officials but tolerated the movement's presence on the streets, with thousands of people attending its pro-Morsi vigil and tens of thousands appearing at its demonstrations.
That patience seems to have run out. Prime Minister Hazem el-Beblawi, head of the interim cabinet installed by the army, said there was growing violence by increasingly well-armed protesters, citing a bomb attack on a police station.
"The presence of weapons, intimidation, fear - this causes concern, especially when there are calls for many to come out tomorrow from different sides," he told a news conference.
After a month nearly 200 people have died in political violence, many fear the protests will lead to more bloodshed.
Past incidents of violence have tended to run through the night and into the following day. Another security official forecast clashes beginning Friday night and stretching into Saturday, the period covered by the army's ultimatum. He also indicated that the two-day period was expected to be decisive.
"The history of Egypt will be written on those days," said the official, who asked not to be identified.
Reiterating his group's commitment to peaceful protest, senior Brotherhood politician Farid Ismail accused the security services of readying militias to attack Morsi supporters, adding that Sisi aimed to drag Egypt into civil war.
"His definition of terrorism is anyone who disagrees with him," Ismail told Reuters. "We are moving forward in complete peacefulness, going forward to confront this coup."
Brotherhood leader Mohamed Badie accused Sisi of committing a crime worse than destroying the Kaaba - the site in Mecca to which all Muslims face when they pray - "brick by brick".
But many Egyptians are no less passionately backing the army, determined to see the Brotherhood reined in.
"There are men carrying guns on the street ... We will not let extremists ruin our revolution," said Mohammed Abdul Aziz, a spokesman for Tamarud, an anti-Morsi petition campaign that mobilized protests against his rule.
"Tomorrow we will cleanse Egypt," he told Reuters.
Sisi's speech on Wednesday pointed to the deepening confrontation between the Brotherhood and the military establishment, which has reasserted its role at the heart of government even as it says it aims to steer clear of politics.
Saying it moved against Morsi in response to the biggest popular protests in Egypt's history, the army installed an interim cabinet that plans to hold parliamentary elections in about six months, to be followed by a presidential vote.
The Brotherhood says it wants nothing to do with the transition plan. With Morsi still in military detention at an undisclosed location, there is slim hope for compromise.
Egypt remains deeply split over what happened 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.
Sisi announced the nationwide rallies after the bombing of a police station in Mansoura, a city north of Cairo, in which a policeman was killed. The government called it a terrorist attack. The Brotherhood also condemned the bombing, accusing the establishment of seeking to frame it.
Since Morsi was deposed, hardline Islamist groups have intensified a violent campaign against the state in the lawless Sinai Peninsula, with near-daily attacks on the police and army.
Two more soldiers were killed on Thursday in an attack on a checkpoint, security and medical sources said.
At the Brotherhood protest camp near a Cairo mosque, Morsi supporters said they expected the army to provoke violence to justify its crackdown. "The army itself will strike. They will use thugs and the police," said medical student Sarah Ahmad, 24.
Essam wl-Erian, another senior Brotherhood politician, accused "the putschists" of trying to recreate a police state, telling a televised news conference: "This state will never return, and Egypt will not go backwards."
Additional reporting by Tom Perry and Maggie Fick, Noah Browning, Tom Finn, Shadia Nasralla, Asma Alsharif and Omar Fahmy; Writing by Tom Perry and Matt Robinson; Editing by Peter Graff and Alistair Lyon