The Muslim Brotherhood, an officially banned movement seen as the best organised opposition, issued a statement on Thursday calling for a national unity government to replace Mubarak.
The Islamist group, whose potential rise to power troubles Egypt's Western allies, had so far taken a back seat in the protests that have swept through the country.
Here are some facts about the Muslim Brotherhood, which has links with Islamist movements in several other Arab countries.
-- Schoolteacher Hassan al-Banna founded the Brotherhood in the Suez Canal town of Ismailia in 1928, partly in response to the British occupation of Egypt. It was one of the first and most successful movements advocating Islam as a political programme in a modern context.
-- Within 20 years the movement had grown to more than 500,000 members, with several branch movements in other Arab countries.
-- The Brotherhood once had a secret paramilitary section but it now says it is committed to promoting its policies through non-violent and democratic means.
-- Mubarak and his government, which sees the Brotherhood as the greatest threat to its survival, has failed to prove any serious act of violence by the movement's leadership for more than 50 years.
-- Brotherhood leaders have argued for social and economic reforms and given the freedom to choose most Egyptians would willingly embrace a form of Islamic law.
-- The government has repeatedly denied the Brotherhood the right to form a political party, arguing the constitution, which the government wrote, bans religious parties. The Brotherhood in turn has said it will not seek recognition as a party under procedures which it rejects as authoritarian.
-- The government banned the Brotherhood in 1954 after accusing the group of trying to assassinate President Gamal Abdel Nasser, a charge the Brotherhood has always denied.
-- A long period of repression began to ease under President Anwar Sadat in the 1970s. The ban formally remains in place but the Brotherhood operates openly within limits that vary at the whim of the authorities.
-- In Egypt the participation of the Muslim Brotherhood in parliamentary elections in the 1980s was followed by its boycott of the elections of 1990, when it joined most of the country's opposition in protesting electoral restrictions.
-- In the 2000 elections Brotherhood supporters running as independent candidates were able to win 17 seats, making it the largest opposition bloc in parliament.
-- In the 2005 parliamentary elections, members won one fifth of the seats in parliament, more than any opposition group has held since the overthrow of the monarchy in 1952.
-- However last December the movement said it would shift its political struggle to the streets after November 2010 elections that were "rigged" to ensure it was ejected from parliament.
-- Mubarak's ruling National Democratic Party (NDP) won 420 of the 508 seats contested in the elections on Nov. 28 and Dec. 5. The Brotherhood had held 88 seats in the previous parliament, giving it a platform to attack official policy. It pulled out of the elections after winning no seats in the first round.
-- In the absence of systematic polling in Egypt, no one has a clear idea how much popularity the Brotherhood enjoys. But the group does have an extensive and well-organised network of committed organisers and has won public support through the charitable work of its professional members. They are also influential in professional organisations such as the doctors' and lawyers' syndicates.
-- Government and ruling party officials had been looking for legal ways to reduce the political role of the Brotherhood, official sources have said. But in the absence of a workable plan, the government has relied on the police to disrupt the movement's activities. Members can expect to be detained for long periods without trial or charge, especially when elections are imminent.
-- The Brotherhood's main foreign ally is the Palestinian Islamist movement Hamas, which has its origins in the group's Palestinian branch. While non-violent at home, the Brotherhood supports the right to armed resistance to Israeli occupation of Palestinian territory. (Additional writing by David Cutler, London Editorial Reference Unit) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/)
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