CAIRO (REUTERS) - Abdel Fattah al-Sisi's last main challenger for the presidency pulled out on Wednesday (Jan 24), saying any hope of a new start for Egypt had ended in a race that is all but certain to deliver the leader a second landslide.

Former military commander Sisi was elected in 2014, a year after he led the army to oust Islamist President Mohamed Mursi.

After rights lawyer Khaled Ali's withdrawal on Wednesday and the arrest of the only serious challenger, former military chief of staff Sami Anan the day before, the March 26-28 vote now faces the prospect of featuring only one candidate.

Ali announced he was withdrawing from the race just hours after Sisi launched his own bid. He said his supporters were being intimidated and the electoral process was corrupt.

"People's confidence in the possibility of transforming electoral gains into a chance for a new beginning, has unfortunately, in our view, for now ended," Ali said at a news conference in central Cairo.

"We announce our decision not to enter this race."

Ali said there was "stubbornness by the electoral commission and many violations" in favour of Sisi.

The electoral commission has said it will ensure the vote is fair and transparent. Egypt's president's office and government press centre have not commented on the election race.

Other candidates have until Jan 29 to register before a final list is announced on Feb.20. A small crop of lesser-known challengers have announced their intention to run but it is far from certain they will garner the necessary nominations required under Egyptian law to actually run.

Egypt, one of the top recipients of US foreign aid, was rocked by unrest that followed mass protests in 2011 which ousted Hosni Mubarak, one of four Arab leaders to be toppled by a series of uprisings across the region.

Supporters say Sisi has brought more security since 2013, although the country faces a stubborn Islamic State in Iraq and Syria (ISIS) insurgency in the North Sinai region. Critics say his popularity has been damaged by austerity reforms and a crackdown on dissidents.

Ali's announcement came a day after Egyptian authorities detained the last challenger seen as a real threat to Sisi's re-election, ex-military chief of staff Lieutenant-General Anan.

The army accused Anan of breaking the law by running for office without permission and his campaign was halted in its tracks.

'BRAZEN ATTACK'

Anan's son, Samir, said police had raided the family home late on Tuesday.

"They took everything they found in his office... they stayed until about 1am," he said.

The United Nations said it was concerned about reports of Anan's arrest. "We urge the authorities in Egypt to ensure that the election campaign is conducted in a credible, inclusive, peaceful and participatory manner," UN spokesman Stephane Dujarric told reporters in New York.

Amnesty International said on Wednesday this was a "brazen attack" on freedom of expression and political participation.

"Anan is among a growing number of candidates arrested or convicted on trumped up charges by the Egyptian authorities,"the London-based rights group said in a statement.

Egypt was "hell-bent" on arresting and harassing anyone who stood against Sisi, Amnesty said.

Candidates challenging Sisi have described sweeping efforts to kill off their campaigns before they begin, with media attacks, intimidation of supporters, and a nomination process stacked in favour of the former general.

When he announced last week he intended to run, he listed Egypt's achievements in his first term and said: "Building the state takes 16 to 20 years, I am trying to finish it in eight years, God willing."

Sisi has vowed to crush Islamist militants and revive a battered economy. Tough fiscal reforms introduced over the past year, in line with a US$12 billion International Monetary Fund loan, hit Egyptians hard but has attracted new foreign investors and economists expect growth to pick up.

The ISIS insurgency in North Sinai has expanded to include civilian targets in the past year. Egypt this month renewed its state of emergency for three more months, broadening the power of authorities to crack down on what it calls enemies of the state.

Sisi addressed police officers on the eve of annual Police Day, which is also the date that Egypt's 2011 uprising began.

"Stick to the goal... we are talking about construction and development. No one (should) make us go astray," he said.