Hosni Mubarak's former intelligence chief Omar Suleiman began the application process on Saturday to enter Egypt's presidential race, while other politicians battled to keep their candidates from being disqualified on technical grounds.
The moves have shaken up Egyptian politics and raised passions one day before the deadline on Sunday for candidates to nominate themselves in the country's first free presidential vote, scheduled to take place in two rounds in May and June.
To many of those who led the uprising, Suleiman's reappearance is proof that a powerful security establishment is determined to reverse a transition to democratic rule before the army hands power to a civilian president.
In a statement circulated by his campaign aides, Suleiman said public demand had persuaded him to run if he could obtain the necessary registration of 30,000 supporters by Saturday.
The statement to citizens of Egypt said: I have been shaken by your strong position. The call you have directed is an order and I am a soldier who has never disobeyed an order.
Later on Saturday, Egypt's electoral commission said it had received verification that the mother of leading Salafi presidential candidate Hazem Abu Ismail had U.S. citizenship, a status that would likely disqualify him from the race.
Abu Ismail has emerged as one of the frontrunners.
The commission said it had received a letter from the Foreign Ministry informing it that Nawal Abdel-Aziz, mother of Hazem Abu Ismail, obtained American nationality on October 25, 2006, the commission's Hatem Begato told Reuters.
The commission would give a verdict on Abu Ismail's eligibility after the Sunday deadline for all presidential candidates to submit their applications to run, he said.
His campaign denied in an emailed statement on Saturday that his mother had any citizenship other than Egyptian, accusing the United States of having presented forged documents to the Egyptian government. He would appeal any decision to bar him from running, it said.
Thousands of his supporters demonstrated the day before against what they called an official plot to stop the ultraconservative sheikh contesting the vote.
On Saturday, the Muslim Brotherhood's Freedom and Justice Party (FJP), which is also contesting the presidential vote, said it would put forward another candidate in reserve in case its first candidate, businessman Khairat al-Shater, was disqualified.
The FJP holds nearly half the seats in Egypt's recently elected parliament.
Shater was convicted in 2007 by Mubarak's government of having provided military training to university students, a charge he denies. Egypt's ruling military council has not pardoned him, which could disqualify him from running.
The FJP's leader Muhamed Mursi, will submit an application to enter Egypt's presidential race. Mursi will act as a backup candidate in case Khairat Shater's candidacy is rejected, a member of Shater's campaign told Reuters.
Should Shater's documents get accepted, Mursi will pull out of the race. If Khairat is rejected, Mursi will continue in the race, Shater's campaign official said.
A Cairo court on Saturday barred yet another candidate, liberal Ayman Nour, from running, saying he had been accused in a case and was still in the process of getting a pardon on health grounds.
Nour came a distant second to Hosni Mubarak in a 2005 election, but a few months later was sentenced to five years in prison on charges of forgery that were widely viewed as trumped up as part of a political vendetta.
In late March, Nour said the country's military rulers had lifted the ban preventing him from running for the presidency.
Suleiman, long viewed as the power behind his ousted boss, said overwhelming popular demand prompted his decision to become a candidate. He picked up his application papers on Saturday ahead of the Sunday deadline for their formal submission.
Like Mubarak, whose three-decade rule ended in February 2011 in a popular uprising, Suleiman has kept far from the public gaze during the past year of turbulent military rule.
Mubarak appointed Suleiman as his vice president in the dying days of his administration, one of several failed concessions to stem the revolt against poverty, corruption and draconian security control.
Suleiman's shadowy persona and his call during the revolt for protesters to go home make him anathema to the young revolutionaries pressing for a new era of accountability and transparency.
The youth will not let Omar Suleiman become president. The revolution is still alive and we will march to Tahrir Square again if necessary, said Mohamed Fahmy, a revolutionary socialist who played a role in galvanising last year's protests.
The very idea that he is running is presumptuous. He should be in prison, said democracy activist and commentator Nawara Negm.
Activists have poked fun at Suleiman's candidacy, saying his campaign had adopted the slogan You are all Khaled Said, a mock reference to the Facebook group whose page We are all Khaled Said helped kindle the uprising that ousted Mubarak.
Police in Alexandria are alleged to have been beaten the 28-year-old Said to death in 2010 for having posted an Internet video purportedly showing two policemen sharing the spoils of a drug bust.
The return of Suleiman, the man seen by many Egyptians as the mastermind of Mubarak's autocratic rule, comes as discontent grows over the insecurity that has endured since his removal.
The economy is still reeling from the turmoil of the uprising, and Coptic Christians and secularists are alarmed at the growing political dominance of Islamists, who were repressed by Mubarak.
Hundreds of Suleiman supporters staged a rally in Cairo on Friday carrying banners reading Suleiman, save Egypt and We don't want the Islamists.
A senior FJP official said the army and Mubarak-era remnants had been bussing thousands of company employees to Cairo to provide many of the 30,000 signatures Suleiman needs to be a registered candidate.
He is the old regime and would only run the country from a security perspective, said the FJP official, Medhat Hadad. What kind of a revolutionary vision do you expect someone like him to have?
Hadad said he believed the army was openly supporting Suleiman's candidacy to cast its real preferred candidate, liberal nationalist and former Arab League chief Amr Moussa, in a better light.
In a poll in March, before Shater and Suleiman emerged as candidates, Moussa was frontrunner with hard-line Salafi Islamist candidate Hazem Salah Abu Ismail in second place and Mubarak's last Prime Minister Ahmed Shafiq in third.
Shafiq welcomed Suleiman's candidacy in a statement saying it represents an expression of the richness of the current civilian trend that desires to protect the state's Egyptian identity.
(Additional reporting by Patrick Werr, Dina Zayed and Shaimaa Fayed; Writing by Tom Pfeiffer and Patrick Werr; Editing by Myra MacDonald)