Former Gen. Sami Annan became the latest in a series of potential contenders who have dropped out or been driven from the presidential campaign.
El-Sissi, who submitted his formal candidacy documents earlier Wednesday, was always considered virtually certain to win a second, four-year term no matter who ran against him. But Annan could have drawn a substantial protest vote, bolstered by his own army credentials and his calls for political inclusion. Egypt's president's office and government press centre have not commented on the election race. Unless someone steps forward by Monday, the deadline to enter the race, el-Sissi could be the sole candidate. Ali, himself, was fighting a court conviction for making a rude gesture that could have disqualified him. The next appeal hearing is due in early March.
"All options are open, including quitting the race", he said.
The armed forces said that Anan's statement regarding running for president contained clear incitement against the armed forces for the objective of driving a wedge between the armed forces and the great Egyptian people.
The court also accused, in a statement, the 69-year-old former general of forging official documents to end his military service and had not obtained the army's approval to run.
Also Wednesday, Amnesty International said Annan's arrest was an attack on rights to public participation and freedom of expression.
"The way the state dealt with Anan is of greater concern than whether his bid was actually a violation of military law", said Mr Ali's campaign spokesman Amr Abdel Rahman.
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"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.
If charged, Anna could face court-martial.
It was not clear what the incitement allegation referred to.
The security forces, meantime, are confronting an Isis (Islamic State) affiliate, based in the North Sinai province, that has steadily killed police and soldiers and staged numerous attacks on civilians throughout the country. "But in terms of outward-facing decisions, el-Sissi and the regime still exert top-level control".
The only remaining well-known potential candidate is now Mortada Mansour, the head of Egypt's legendary Zamalek football club.
Mr Sisi came to power after leading a popularly backed military coup in 2013 against his elected Islamist predecessor, Mohamed Morsi, a leader of the Muslim Brotherhood. Morsi then elevated el-Sissi to defense minister. Under el-Sissi, links to the Brotherhood are grounds for prosecution. Former lawmaker Mohammed Anwar Sadat previously announced he was dropping out of the race, partly out of fear for the safety of his supporters.
Anan was detained as he drove through the Nasr City neighborhood of Cairo, his son Samir said by telephone.