The Egyptian army has thrown its weight behind anti-Morsi protesters with the warning that "if people's demands are not met in 48 hours" it will present "measures to be implemented by all parties with no exclusion".
Gen Abdel Fattah al-Sisi described the protests calling for the president to be ousted as "glorious" but said the army would "not be a party in politics or rule".
It had a responsibility to act because Egypt's national security was facing a "grave danger". according to the statement, read out on state television.
Meanwhile, Egyptian president Mohamed Morsi has rejected the resignation of four ministers from his Islamist government.
The four - Hisham Zaazou, Atef Helmi, Hatem Bagato and Khaled Abdel-Aa - presented their resignation in sympathy with the protesters.
Gehad El-Haddad, spokesman of the Islamist movement, told Reuters that Egyptians would not sit by and tolerate attacks on their institutions.
"It's very dangerous for one entity in society to take up violence as a means of change because it may entice others to do so. The Muslim Brotherhood is a disciplined organisation," he said, criticising the security forces for failing to protect the headquarters in Sunday's attack.