April 6 Democratic Front started a sit-in near the presidential palace late Sunday to demand the release of its member Abdel-Rahman Mohsen.
Mohsen is accused of being a founding member of the outlawed radical opposition group Black Bloc.
The sit-in will continue until Mohsen's trial begins on Tuesday, the group said.
The youth group’s spokesperson Mustafa El-Hagry told state news agency MENA that the protesters do not plan any confrontations with security forces at the palace and the sit-in would be limited to the area around Heliopolis Club, opposite the palace.
The youth group had attempted to hold the sit-in directly in front of the palace but security forces stopped them.
Since the beginning of the year, around 2,000 people have been arrested during clashes in Cairo, most of whom have been released but not formally acquitted. Dozens have been arrested from their homes for allegedly being members of Black Bloc, the radical opposition group.
Also, several well-known activists have been put on trial during the past few months, including April 6 founding member Ahmed Maher, blogger Alaa Abdel-Fattah, No to Military Trials founding member Mona Seif, as well as Justice and Freedom Youth member Ahmed Doma, who remains in custody.