His critics say he lacks the political experience to lead Egypt, so 71-year-old Mohamed ElBaradei will need to draw on a past of international diplomacy, largely made up of advising the UN on nuclear weapons.
He won a Nobel Peace prize during his 12 years as director-general at the International Atomic Energy Agency, after succeeding Hans Blix. He held the role from 1997 during which time he was drawn into rows about nuclear capacity in Iran and Korea. He has spoken about the difficulty of nations equipped with nuclear arms telling other countries they could not do the same. ElBaradei was critical of the allied invasion of Iraq 10 years ago, recently calling the war a "global act of deception" with a "colossal regard for civilian victims".
His education was steeped in law. He was born in Cairo, the son of an Egyptian lawyer, Mostafa ElBaradei, who campaigned for greater democracy and a free press. Later, he studied at the University of Cairo, the Institute of International Studies in Switzerland, and then for a doctorate in international law at the New York University School of Law. The father of two can speak fluent French and English. On Twitter, his recent tweets include: "Egypt is a train wreck waiting to happen" and "I strongly condemn violence in all forms... The more peaceful, the stronger we become."
In 2009 he returned to Egypt as a critic of Hosni Mubarak, and spoke directly to the protesters in Tahrir Square as the ruler was overthrown in 2011. He formed his own party, the Constitution Party, last year. To the distress of Muslim groups, it positions itself as liberal and secular.