The head of the Egyptian military warned political conflict could lead to the collapse of the state and said protecting the Suez Canal was one of the main objectives of the army deployment to nearby cities shaken by violence.
Protesters defied a curfew in towns along the Suez Canal overnight, attacking police stations after President Mohamed Morsi imposed emergency rule to end days of clashes that have killed at least 52 people.
The remarks of General Abdel Fattah al-Sisi, who is also defence minister, were published on the official Facebook page of the army spokesman.
He said the economic, political and social challenges facing Egypt represented "a real threat to the security of Egypt and the cohesiveness of the Egyptian state" and the army would remain "the solid and cohesive block" on which the state rests.
The army, he said, belonged to all Egyptians regardless of their sect or political affiliation.
"The continuation of the struggle of the different political forces ... over the management of state affairs could lead to the collapse of the state," he said.
"The army's deployment in Port Said and Suez provinces aims to protect the vital strategic interests of the state, at the forefront of which is the vital Suez Canal," he said, adding the army would not allow the canal to be harmed.
The military assumed power from deposed President Hosni Mubarak at the height of the uprising against him in 2011 before leading the state through an interim period that formally ended with Mr Morsi's election in June last year.
At least two men died in overnight fighting in the canal city of Port Said in the latest outbreak of violence unleashed last week on the eve of the anniversary of the 2011 revolt that brought down autocrat Hosni Mubarak.
Political opponents spurned a call by Mr Morsi for talks yesterday to try to end the violence.
Instead, huge crowds of protesters took to the streets in Cairo, Alexandria and in the three Suez Canal cities - Port Said, Ismailia and Suez - where Mr Morsi imposed emergency rule and a curfew on Sunday.
"Down, down with Mohamed Morsi! Down, down with the state of emergency!" crowds shouted in Ismailia. In Cairo, flames lit up the night sky as protesters set police vehicles ablaze.
In Port Said, men attacked police stations after dark. A security source said some police and troops were injured. A medical source said two men were killed and 12 injured in the clashes, including 10 with gunshot wounds. "The people want to bring down the regime," crowds chanted in Alexandria. "Leave means go, and don't say no!"
Mr Morsi and his supporters accuse the protesters of seeking to overthrow Egypt's first ever democratically elected leader through undemocratic means.
In Cairo yesterday, police fired volleys of teargas at stone-throwing protesters near Tahrir Square, cauldron of the anti-Mubarak uprising. Demonstrators stormed into the downtown Semiramis Intercontinental hotel and burned two police vehicles.
A 46-year-old bystander was killed by a gunshot early yesterday, a security source said. It was not clear who fired.
Mr Morsi's invitation to opponents to hold a national dialogue with Islamists yesterday was spurned by the main opposition National Salvation Front coalition, which rejected the offer as "cosmetic and not substantive".
Reuters