Security forces have put up barbed wire and formed a human chain to contain supporters of the ousted Egyptian president at a rally in front of the Rabaa mosque in Cairo.
Some demonstrators said they would die before allowing him to be removed from office.
One supporter told Reuters news agency: "God willing we will not leave here until the president we voted for under the name of God and his prophet is reinstalled in office, or else we shall become martyrs."
The Muslim Brotherhood tells ITV News they are against violence but will "pay the price of their blood" to restore their ousted president.
Egypt's army has named two further officials in the transitional government as its supporters and opponents vowed to stay on the streets.
There has been outrage in Egypt after 51 people were killed near an barracks where ousted President Mohamed Morsi is thought to be held.