The Egyptian president, Mohamed Morsi, has said he will not step down as demanded by millions of protesters, vowing to protect his "constitutional legitimacy" with his life.
Addressing the nation in a speech carried live on state television late on Tuesday, Morsi accused loyalists of his predecessor Hosni Mubarak of riding the current wave of protests to topple his regime.
"There is no substitute for legitimacy," said Morsi, who has received an ultimatum from the military to work out his differences with the opposition by Wednesday or it will intervene to oversee the implementation of its own political road map.
Morsi demanded earlier that the army withdraw an ultimatum to resolve the nation's political crisis, saying that he will not be dictated to.
Morsi insisted on his "constitutional legitimacy" on his Twitter account, earlier on Tuesday night, hours after the army published a plan to dissolve parliament, rewrite the constitution and hold new elections if he could not end protests against his rule by Wednesday.
Morsi faces huge pressure to resign from his position, and a warning from the military that politicians must resolve the crisis or face the army's transitional "roadmap".
Four people were killed amid a third night of protests in Egypt. A health ministry source told Al Jazeera that they were killed in clashes in Giza - three were shot dead at a pro-Mosri demonstration in Nahda Square near Cairo University, and another was killed by gunfire while taking part in march.
The Muslim Brotherhood office in Helwan was reported to have been ransacked and set on fire.
Al Jazeera's Sherine Tadros, reporting from Tahrir Square, said: “People here have no doubt the army is on their side... and as the military’s deadline approaches their screams demanding Morsi resign are growing louder and the window of opportunity to offer concessions is closing.”
Opposition groups, such as the Tamarod, have called on supporters to remain on the streets until the deadline has passed on Wednesday.
Pressure on Morsi continued to mount as the UN high commissioner for human rights, Navi Pillay, called on him to engage in "serious national dialogue" with his opponents.
US President Barack Obama contacted Morsi by telephone to urge him to listen to all Egyptian voices, "including the many Egyptians demonstrating throughout the country".
More of Morsi's cabinet and advisers resigned on Tuesday, with foreign minister Mohamed Kamel Amr following five others out of the door.
The president also lost the support of Sami Enan, his military adviser, who resigned and said the army would not "abandon the will of the people".
Committed to democracy
|Follow spotlight coverage of the struggling young democracy
In an interview with Al Jazeera, Alaa Moustafa, a spokeswoman for the Freedom and Justice Party, the political wing of the Muslim Brotherhood, said the party was committed to democracy and the president would call for a national dialogue to let every party take part.
When asked if the party was fully behind Mohamed Morsi as president, she said: "The Freedom and Justice Party and the Muslim Brotherhood are behind democracy... we are here to support the Egyptian will."
Asked the question again, she replied: "We are behind the will of the people. In democratic countries are known ... by elections. The only way to change is not through protestation. We have a democratic path to go through."
"We are totaly against any person trying to get our country back to dictatorship.... We are with any initiative that takes things back to normal… as long as it is under the umbrella of the constitution. We are with any solution."
The army on Tuesday also reiterated that its initial statement on Monday was not a de facto "coup".
"The doctrine and culture of the Armed Forces do not allow the adoption of any 'military-coup-based' policies. The Egyptian military always stands by the will and aspirations of the glorious Egyptian people for change and reform.
"The aim of the Armed Forces' statement is to push all political parties nationwide to quickly find solutions for the current crisis and reach a formula of national compromise that complies with the demands of the Egyptian people."
The president was due to issue a statement later on Tuesday, but has not spoken directly on the crisis in days.
|Hoda Abdel-Hamid reports from Tahrir Square
In a statement issued nine hours after the army deadline was imposed, the president's office said: "The president of the republic was not consulted about the statement issued by the armed forces.
"The presidency sees that some of the statements in it carry meanings that could cause confusion in the complex national environment."
Morsi met the head of the armed forces, General Abdel Fattah al-Sisi, for a second day, the president's office said later on Tuesday.
In response to the army deadline, Muslim Brotherhood spokesman Mohamed al-Beltagui urged Morsi’s supporters to stand ready to sacrifice their lives to prevent a coup.
"Seeking martyrdom to prevent this coup is what we can offer to the previous martyrs of the revolution," he said in a statement, a reference to more than 800 people killed during the 2011 uprising against Hosni Mubarak.
The protests have drawn millions of people onto the streets, with a majority of them demanding Morsi's resignation.
The June 30 Front, an opposition umbrella group which includes Tamarod, on Tuesday said it had chosen Mohamed ElBaradei to represent it at any politicial transition.