Cairo, Egypt (CNN) -- With the rights of several candidates in question before Sunday's deadline to qualify and an 11th hour entrance of the former spy chief, Egypt's already complex and convoluted presidential election stands to be upended.
Candidates were required to submit documents by the end of Sunday.
Elections are scheduled to start May 23.
Omar Suleiman, the intelligence chief of deposed strongman Hosni Mubarak, formally entered the race just hours before the deadline, said Hatem Bjato, who heads the election committee.
Suleiman had said he would not seek the presidency in the first election for the post since the revolution that led to Mubarak's exit. But on Friday, he did an about-face, saying he felt obliged to supporters.
He collected the signatures of 30,000 eligible voters needed to qualify, Bjato said.
"I promise you, my brothers and sisters, to complete the goals of the revolution and provide security and stability to the Egyptian people," he said in a written statement Friday.
Other candidacies were less certain.
A court ruled that liberal opposition leader Ayman Nour will not be allowed to compete because he was jailed in recent years, the candidate's son said Saturday. Nour was recently pardoned and plans to appeal, the son said.
The decision potentially cast doubt on the future of another candidate, Khairat al-Shater, the nominee of the once-banned Muslim Brotherhood who was also pardoned for his past convictions.
A millionaire businessman who served two prison terms under former President Mubarak, al-Shater is considered a conservative, though he is also credited as being the driving force behind the Brotherhood's affirmation that Egypt should continue to honor its international agreements -- including its peace treaty with Israel.
Fearful for the future of its candidates, the Muslim Brotherhood nominated Saturday an alternative, Mohammed Morsi, chief of the Freedom and Justice Party, the group's political arm.
"We are protecting the revolution and all of its goals ... We have decided as the Brotherhood and its party to nominate Mohammed Morsi as our backup candidate for president," it said in a statement.
The group had pledged repeatedly that it would not field a presidential candidate. But candidates from its political arm won the largest share of seats in Egypt's parliamentary elections in December. And Brotherhood leader Mohamed Badie has said the new Egypt "is under a serious threat" because its current, military-led government "has failed to represent the will of the people."
A military junta took power after Mubarak's ouster.
Salafist candidate Hazem Abu Ismael was also disqualified from running in the election because of his mother's U.S. citizenship, state television reported Saturday.
The deceased mother of Abu Ismael held U.S. citizenship and used her U.S. passport to enter Egypt three times, Egypt's Interior Ministry has said.
Ismael had said that his mother held a green card residency permit but was not a U.S. citizen. He told a private Egyptian TV program that his sister was married to an American and had obtained U.S. citizenship, but that his mother had not.
More than 450 people have already registered or announced plans to seek the presidency.