Egyptian police arrested on Wednesday a leading government critic and party leader Abdel Moneim Abul Fotouh, security officials said, while prosecutors remanded into custody another leading dissident.
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 on Thursday.
The arrests came after a statement by Abul Fotouh and several other politicians calling for a boycott of next month's election.
Meanwhile, the country's 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 registering 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.
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 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.
Source: News Agencies, Edited by website team