A detained member of the Nasserist Popular Current has announced that he will begin a hunger strike after his detention was renewed this week for a further 15 days.
Omar El-Kady was arrested on 25 January, the third anniversary of the 2011 revolution.
According to interior ministry figures more than 1,000 people were arrested in protests and clashes with security forces that took place the day, while at least 64 people were killed.
The vast majority of those arrested remain in detention pending investigation.
The Popular Current has said that the families of the detained group members will hold a press conference in the coming days to announce that they will also go on hunger strike in solidarity with those detained.
According to a report by Amnesty International, Egyptian security forces made a "staggering" number of arrests and ill-treated detainees, including children, on 25 January.
Interim President Adly Mansour has called on the prosecutor-general to look into the status of those detained pending investigation, university students in particular.