Foreign Secretary William Hague said he is "concerned" about the situation in Egypt.
Mr Hague wrote on Twitter that it is "vital it is resolved peacefully":
Concerned about situation in #Egypt. Vital it is resolved peacefully and in a way that supports the democratic transition
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.