Huge crowds of Algerians demonstrated against the political establishment in the capital on Friday and denounced the arrest the previous day of a leading figure of a months-long protest movement.
Abdelouahab Fersaoui, who heads the civic group Youth Action Rally (RAJ), was arrested on Thursday during a protest in support of detainees.
"We have had no other news of him" since Thursday afternoon when he phoned to say he was being held in a police station, Said Salhi, vice president of the Algerian League for the Defence of Human Rights, told AFP.
The RAJ has been a key player in the anti-regime protest movement that has rocked Algeria since February.
Demonstrators are now demanding political reforms and the removal of regime insiders before a presidential vote to replace longtime leader Abdelaziz Bouteflika, who resigned in April after mass protests.
Security forces have toughened their line against the weekly protests, particularly after acting president Abdelkader Bensalah last month announced a December 12 date for the election.
They have blocked protesters from outside Algiers from reaching the capital and carried out several arrests.
The National Committee for the Liberation of Detainees says that more than 80 people are being held by the authorities and has demanded the release of "political prisoners and prisoners of opinion".
On Friday, demonstrators in Algiers chanted: "Take us all to jail. The people will not back off," while others held up a huge banner bearing the portraits of several activists, journalists, students or ordinary citizens arrested in recent months.
"I am not afraid. I protest every Tuesday and every Friday and I am willing to keep at it... If they have enough room in prison they can take all of us in," Akli, a 68-year-protesters, said.
The demonstrators also branded the police as "oppressors of students" for having prevented on Tuesday a weekly protest by students in the capital for the first time since February.
Others vented their fury against powerful army chief General Ahmed Gaid Salah, who has led the push for presidential polls by the end of 2019, saying it was the only way out of the crisis.
"Gaid Salah. No vote this year," they shouted.