Tens of thousands of rival Egyptians poured onto the streets on Wednesday, minutes before a controversial army deadline to impose a political solution after a defiant Islamist President Mohamed Morsi vowed to see out his term regardless.
The interior ministry warned that police would respond firmly to any violence after a week of bloodshed -- which has now killed almost 50 people -- intensified as Morsi supporters and opponents again squared off overnight.
Morsi's opponents accuse him of having betrayed the revolution by concentrating power in Islamist hands and of sending the economy into freefall.
His supporters say he inherited many problems, and that he should be allowed to complete his term, which runs until 2016.
As the clock ticked down on the army's deadline for Morsi to meet the "people's demands" by 4:30 pm (1430 GMT), General Abdel Fattah al-Sisi held talks with top brass, a source close to the army told AFP.
Sisi, who is also the defence minister, later went into a meeting with opposition leader Mohamed ElBaradei, Coptic Christian Patriarch Tawadros II and Sheikh Ahmed al-Tayeb, the grand imam of Al-Azhar, Sunni Islam's highest seat of learning.
Also present were representatives of the Salafist Al-Nur party and the Freedom and Justice Party, the political wing of President Morsi's Muslim Brotherhood.
The crunch talks came as thousands of protesters opposed to Morsi massed in Cairo's Tahrir Square, epicentre of the 2011 uprising, after the Islamist leader delivered a televised address to the nation rejecting calls for him to step down.
In his defiant speech, Morsi said he had been freely elected to lead the troubled nation a little more than a year ago and intended to stick to his task. The only alternative was more bloodshed, he warned.
Upping the stakes, senior armed forces commanders meeting on Wednesday swore to defend Egypt with their lives, a source close to the military told AFP.
"We swear to God that we will sacrifice our blood for Egypt and its people against all terrorists, extremists and the ignorant," they declared in an oath led by army chief Abdel Fattah al-Sisi, the source said.
The army will issue a statement after the deadline passes, said the military source, without giving a specific time.
As the two camps upped the stakes, the health ministry reported unidentified gunmen had killed 16 people and wounded about 200 more when they opened fire on a rally by his supporters in Cairo overnight.
Egypt's press predicted Wednesday would be the day of Morsi's departure.
"Today: Ouster or Resignation," splashed the state-owned mass circulation Al-Ahram. "The End," declared the independent Al-Watan.
Cairo's streets were unusually quiet Wednesday, with many choosing to stay home over fears of more violence.
"The Islamists declared war on the rest of the population yesterday. I'm very scared," said resident Soha Abdelrahman.
Developments in Egypt hit world oil prices, with New York crude hitting a 14-month peak on concerns that the crisis could affect the rest of the Middle East and disrupt supplies, analysts said.
In Asia, New York's main contract, West Texas Intermediate (WTI) light sweet crude for August, closed at $101.07, up $1.45 from Tuesday's close, after briefly surging to $102.18 per barrel -- which was last seen on May 4, 2012.
After Morsi's speech, the opposition Tamarod movement which is also involved in Wednesday's talks with Sisi, mobilised millions of demonstrators at the weekend for what the military described as the biggest protests in Egypt's history, accused Morsi of "threatening his own people".
Morsi's opponents have welcomed the army's 48-hour ultimatum, which was followed by a spate of resignations from his cabinet.
But his supporters accuse the generals of preparing a return to the unpopular military rule of the months between the overthrow of former president Hosni Mubarak in February 2011 and Morsi's swearing-in on June 30 last year.
Government daily Al-Ahram reported details of the army's demands.
Its plan provides for an interim administration, of up to one year, which would include the head of the supreme constitutional court and a senior army figure.
The constitution, controversially approved by Morsi's Islamist allies in December, would be suspended for up to 12 months while a new one was drawn up and put to a referendum, before presidential and legislative elections.
The main opposition June 30 Front coalition said it was ready to join urgent talks on the negotiated transition called for by the army.
US President Barack Obama, whose government is a major military aid donor to Egypt, called Morsi to warn him the voices of all Egyptians must be heard.