Dozens of activists, relatives and friends of Alaa Abdel-Fattah have gathered at Cairo's High Court to demand his release.
The prominent activist has been detained for 100 days as of Sunday, but his trial date is yet to be set.
He was detained for organising an unauthorised protest against military trials of civilians.
Protesters also demand the release of Ahmed Abdel-Rahman, who has been detained on the same charges for 102 days, and all other political detainees.
The protest was disrupted by people chanting "traitors, traitors," "Egypt is more important" and "El-Sisi is my president," referring to army chief Abdel-Fattah El-Sisi.
Abdel-Fattah's relatives and human rights lawyers presented a complaint against the public prosecutor to the Supreme Council of the Judiciary.
"The public prosecutor is ignoring serious complaints [about the detention of Abdel-Fattah and others]. He is being the voice of the state instead of the people," Mona Seif, Abdel-Fattah's sister, told Ahram Online.
Seif said everyone arrested during the military trials protest on 26 November had been released pending trial, except Abdel-Fattah and Abdel-Rahman.
Detainees are usually held for a set period of time, then taken to a judge who decides if they will be held until their trial.
Abdel-Fattah has had his detention period officially extended twice, but since then has remained in jail without seeing a judge or having his trial date set, Seif said.
The family is shocked that such a practice is permitted by Egyptian law, she added.
A founding member of No to Military Trials of Civilians, Seif said some women in the group had told prosecutors they organised the protest, but Abdel-Fattah was still being detained.
Abdel-Fattah's aunt, the novelist Ahdaf Soueif, released a statement on Sunday saying his continued detention was political.
"Alaa is in prison because he is Alaa," the statement read.
Abdel-Fattah was arrested at his home on 28 November, two days after the protest.
In January, he received a suspended one-year jail sentence in a separate case on charges of torching the campaign headquarters of Hosni Mubarak's last prime minister Ahmed Shafiq.