The top judge of Egypt's Constitutional Court, Adly Mansour, has been sworn in as interim leader, hours after the army ousted President Mohammed Morsi and put him under house arrest.
Army chief General Abdul Fattah al-Sisi announced the move on Wednesday, in what Mr Morsi said was a military coup. The General said Mr Morsi, Egypt's first freely elected leader, 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 for the country and of failing to tackle Egypt's economic problems.
The BBC reports Mr Morsi had appeared to protesters to be economically out of his depth, and had not given them the reassurances they wanted that he could address rampant poverty. Some 50 people have died since the latest unrest began on Sunday.
Mr Mansour was appointed chief justice of the Supreme Constitutional Court only four days ago, and was sworn in to that office on Thursday morning. He was then sworn in as interim head of state.
A technocratic interim government will be formed, along with a panel for national reconciliation. The constitution will be reviewed and presidential and parliamentary elections arranged, but no timeframe has been given.
Liberal chief negotiator Mohamed ElBaradei, a former UN nuclear agency chief, said the plan would "continue the revolution" of 2011. Many hope they can have more electoral success than last year, when the Brotherhood's organisation dominated the elections.
United States President Barack Obama has said he is "deeply concerned" by the latest turn of events and called for a swift return to civilian rule.
Morsi detained, others arrested
The army is holding Mohammed Morsi at a military facility in Cairo and other Muslim Brotherhood leaders have been arrested in a crackdown on the movement that won several elections in 2012.
The United Nations, the United States and other world powers did not condemn Mr Morsi's removal as a military coup as to do so might trigger sanctions.
The army's intervention is supported by millions of Egyptians, including liberal leaders and religious figures who expect new elections under a revised set of rules.
But as vast crowds partied in Cairo's Tahrir Square, hailing a "second revolution" to match the overthrow of Hosni Mubarak in 2011, Islamists feared a clampdown that revived memories of their sufferings under the old, military-backed regime.
Mr Morsi, 62, earlier repeated his offer of a consensus government, but refused to step down. He was transported to the Defence Ministry, Brotherhood spokesman Gehad El-Haddad told Reuters. His aides were being held at the Republican Guard barracks where he spent his final day in office defying calls for him to resign but unable to forestall an ultimatum from the generals.
The fall of the first elected leader to emerge from the Arab Spring revolutions has raised questions about the future of political Islam, which only lately seemed triumphant. Deeply divided, Egypt's 84 million people find themselves again a focus of concern in a region traumatised by the civil war in Syria.
Straddling the Suez Canal and a key piece in the security of Israel, many powers have an interest in Egypt's stability.
The army put combat troops and tanks on the streets around a gathering of hundreds of Mr Morsi's supporters in Cairo. The military said it would keep order. Mr Morsi called for there to be no violence. Television stations sympathetic to him have been taken off air.
The clock started ticking for Mr Morsi when millions took to the streets on Sunday to demand that he resign. They accused his Brotherhood of hijacking the revolution, entrenching its power and failing to revive the economy.
That gave armed forces chief General Abdel Fattah al-Sisi, who already had his own reservations about the state of the nation under Mr Morsi, a justification to invoke the "will of the people" and demand the president share power or step aside.
The United States and other Western allies had also pressed Mr Morsi hard to open his administration to a broader mix of ideas.