(Reuters) - Hosni Mubarak´s former intelligence chief formally applied to enter the presidential race on Saturday, shaking up Egyptian politics as liberals and Islamists tried to decipher the motives of a man long viewed as the power behind his ousted boss.

Omar Suleiman, 74, said overwhelming popular demand prompted his decision to become a candidate in Egypt´s first free presidential vote just before nominations close on Sunday.

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.

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 at the end of June.

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 galvanizing 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.

The 28-year-old Said was alleged to have been beaten to death by police in Alexandria in 2010 for having posted an Internet video purportedly showing two policemen sharing the spoils of a drug bust.

A Twitter hashtag message chain about Suleiman was called "Silly Man". Other activists said Egypt´s revolutionary chant "Bread, Freedom and Social justice" would switch to "Bread, Blanket and Prison Food" under Suleiman.

THRONG OF SUPPORTERS

In a statement circulated by his campaign aides, Suleiman said public demand had convinced 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."

Suleiman´s supporters thronged the election committee´s offices in Cairo on Saturday amid tight security as he arrived to begin the paperwork, state news agency MENA reported.

The return of 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 a large Coptic Christian minority 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".

The Muslim Brotherhood, which swept Egypt´s first free parliamentary election in decades, announced a week ago that it was fielding a candidate for the presidency, reversing an earlier pledge not to.

Its candidate, deputy Brotherhood leader Khairat al-Shater, declared last week that introducing sharia law would be his "first and final" goal if he wins the vote in May and June.

On Friday, thousands of supporters of Hazem Salah Abu Ismail - who has emerged as one of the frontrunners for the race - demonstrated against what they called an official plot to stop the ultraconservative sheikh from contesting the election.

"OLD REGIME"

A senior member of the Brotherhood´s Freedom and Justice Party (FJP) 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; Editing by Alistair Lyon) (© Thomson Reuters 2012. 04/07/12)