Nobel Peace Prize winner Mohamed ElBaradei, who has emerged a leading opposition figure in the political upheaval sweeping through Egypt, has rich experience in working at the UN and distinguished himself as the head of the International Atomic Energy Agency (IAEA).
ElBaradei has now returned to Egypt to spearhead the protesters demanding an end to President Hosny Mubarak's 30-year unbridled rule.
A former director general of the UN nuclear watch-dog, ElBaradei told the crowd at Cairo's Tahrir Square on Sunday "what we have begun cannot go back", referring to days of anti-government protests.
ElBaradei was born in Cairo in 1942.
Son of a former president of the Egyptian Bar Association, he gained a Bachelor's degree in law in 1962 at the University of Cairo, and a doctorate in International Law at the New York University School of Law in 1974.
He began his career in the Egyptian Diplomatic Service in 1964, serving on two occasions in the Permanent Missions of Egypt to the UN in New York and Geneva. From 1974 to 1978 he was a special assistant to the foreign minister of Egypt, said nobelprize.org.
In 1980, he left the diplomatic service to join the UN.
During his career as an international civil servant, ElBaradei became closely familiar with the work and processes of international organisations, particularly in the fields of international peace and security and international development.
In 2005, ElBaradei and the IAEA were jointly awarded the Nobel Peace Prize for efforts "to prevent nuclear energy from being used for military purposes and to ensure that nuclear energy for peaceful purposes is used in the safest possible way".
ElBaradei is married to Aida Elkachef, a teacher. They have a daughter, Laila, a lawyer, and a son, Mostafa, a studio director with a television network, both of whom live and work in London.