Under emergency powers announced by President Mohamed Morsi for the cities of Port Said, Ismailia and Suez on Sunday, the army will have the right to arrest civilians and to help police restore order.
The step is likely to anger protesters who accuse Morsi of using high-handed security tactics of the kind they fought against to oust Hosni Mubarak as president.
Although Islamists have won parliamentary and presidential elections, the disparate opposition has since united against Morsi.
Morsi’s opponents accuse him of hijacking the revolution, listening only to his Islamist allies and breaking a promise to be a president for all Egyptians.
They say too many officials from the Mubarak era remain in their posts.
Hundreds of demonstrators in Port Said, Ismailia and Suez, cities which all lie on the economically vital Suez Canal, turned out against Morsi’s decision on Sunday within moments of his speaking. Activists pledged to defy a curfew that starts at 9pm.