Security officials suggested Abul Fotouh, a candidate in the 2012 presidential election, had ties with the outlawed Muslim Brotherhood.
He was arrested shortly after arriving from London where he gave interviews in which he was critical of the government.
Five aides were also arrested, the officials said. Rights lawyer Gamal Eid told AFP that they were subsequently released, but Abul Fotouh remained in custody to be questioned om Thursday.
The arrests came after a statement by Abul Fotouh and several other politicians calling for a boycott of next month’s election.
President Abdel Fattah al-Sisi is expected to easily win the election after most rivals were sidelined or withdrew.
His most credible rival, former military chief of staff Sami Anan, was detained last month shortly after announcing his candidacy. The military said he was still enlisted and had no right to run in the election.
He is accused of fraud for registerting on the voters’ database, and incitement against the military.
Anan’s top campaign aide Hisham Geneina, a former anti-corruption chief sacked by Sisi, was remanded into custody on Wednesday, a day after his arrest, for suggesting Anan possessed and could release documents damning to top state officials.
His lawyer Ali Taha told AFP that he did not know where Geneina was being held, and that he needed medical attention for injuries he sustained when a group of men attacked him in the street days earlier.
Sisi was elected in 2014, a year after the former army chief ousted his unpopular Islamist predecessor Mohamed Morsi.
Abul Fotouh, once a leading member of Morsi’s Muslim Brotherhood, had supported the mass protests against the Islamist that prompted the military to topple him.
He has since been critical of Sisi who has been accused of clamping down on dissent.
Sisi’s only remaining rival is a leader of a small party who had been a leading campaigner for Sisi until he abruptly announced his own candidacy.
Critics said he did so to save Sisi the embarrassment of being the only candidate, in a throwback to referendums held by Egyptian autocrats instead of elections.
The government has rejected such criticism and the candidate, Moussa Mostafa Moussa, insists he is a serious contender.
Former army general and prime minister Ahmed Shafiq had also announced he would stand in the election.
Shafiq made his announcement from exile in the United Arab Emirates, which promptly sent him back to Egypt where he was received by state officials who took him to a hotel.
Shafiq later withdrew his candidacy.