The top judge of Egypt’s Constitutional Court, Adly Mahmud Mansour, has been sworn in as interim leader, hours after the army ousted former President Mohammed Morsi and put him under house arrest.
Mr Mansour said fresh elections were “the only way” forward, but gave no indication of when they would be held.
Mr Morsi, Egypt’s first freely elected leader, is under house arrest after what he says was a military coup.
The army said he had “failed to meet the demands of the people”.
The upheaval comes after days of mass rallies against the Islamist president.
Protesters accused Mr Morsi and the Muslim Brotherhood of pursuing an Islamist agenda and of failing to tackle Egypt’s economic problems.
The health ministry says at least 10 people were killed and scores injured in clashes across the country. Some 50 people have died since the latest unrest began on Sunday.
William Hague has said the UK “will work with the people in authority in Egypt” but condemned the ousting of its president as “a dangerous thing”.
The foreign secretary told the BBC the UK wanted to see a civilian-led government in Egypt.
But he said “we have to work with whoever is in authority” to protect Britons and UK firms in Egypt.