Egyptian prosecution ordered on Saturday the detention of leftist lawyer Haitham Mohamedein for 15 days pending investigation on charges of joining an unlawful group.
However, charges did not specify the name or the nature of the illegal organisation.
Mohamedein, who was arrested on Thursday night from his house, is among tens of activists who were arrested in different parts of Egypt ahead of calls for protests on 25 April.
The prosecution also accused Mohamedein of possession of flyers that call for the overthrow of the regime.
On Thursday night, the police raided several Cairo cafes and arrested dozens of people. Others were picked up from homes in the early hours of Friday morning.
The Ministry of Interior has not issued a public statement on the causes behind the arrests or a tally of those apprehended.
East of Cairo prosecution is currently questioning at least 11 arrested activists and others are expected to appear in Abbasiya court soon, according to rights lawyer Mokhtar Mounib.
Lawyer Amr Imam previously told Ahram Online that at least 100 people were estimated to have been arrested around the country on Thursday night.
Last week, several thousand protestors took to streets to oppose the government’s decision to acknowledge Saudi Arabian sovereignty over the Red Sea islands of Tiran and Sanafir.
Some have vowed to stage another protest on 25 April, which marks Sinai Liberation Day.