Khairat El-Shater, a leader of Egypt's Muslim Brotherhood, registered his candidacy for the country's presidency on Thursday as crowds of supporters cheered him on.
More than 1,000 of his backers chanted "the people want Shater as president" when the Islamist, until recently the Brotherhood's deputy leader, arrived at the election committee's headquarters in northern Cairo.
Last week's announcement of Shater's candidacy sent political shockwaves throughout the post-uprising country as people accused the Brotherhood of trying to monopolise power.
The Brotherhood, which dominates parliament and the senate through its political arm, the Freedom and Justice Party (FJP), had pledged not to field a candidate for the May election.
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But it reversed its position when the ruling military refused to sack the government and replace it with an FJP-led cabinet.
On Wednesday, a Muslim group that met with Shater said he promised to push for the implementation of Islamic law sharia if elected.
A campaign official said Shater shared the FJP's programme, which calls for an Islamic and democratic state.
The election on May 23 and 24 will be the first presidential vote since the ouster of former president Hosni Mubarak in a popular uprising in February 2011 that led to military rule.
The winner will be announced in June, when the military is scheduled to hand over power.
Shater had been jailed under Mubarak, whose government regularly rounded up members of the then-banned Brotherhood, and was released after the dictator's ouster.