UN Secretary General Ban Ki-Moon has appealed for calm, non-violence and dialogue in Egypt.
Military interference in any state's affairs is of concern, he said in a statement that did not overtly criticise the ousting of the president.
"It will be crucial to quickly reinforce civilian rule in accordance with principles of democracy," he added.
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.