Amnesty International called for an "independent" investigation into the allegations.
Algerian police said Sunday they had launched an investigation after videos on social media claiming officers sexually abused a teenage boy arrested during anti-government protests sparked fury.
The 15-year-old was arrested on Saturday in Algiers, the capital of the North African country, where a long-running pro-democracy protest movement known as the Hirak holds regular demonstrations demanding political change.
"After the circulation of a video on social networks in which the authors claim that a minor was mistreated in a police station in Algiers, security forces... informed the public prosecutor, who immediately ordered an investigation," a police statement said.
One video reportedly showed the boy sobbing as he left the police station, while in another, the boy said he was "mistreated" by the police and alleged they "tried to rape him".
Amnesty International called for an "independent" investigation into the allegations.
Last month, the Algerian League for Human Rights (LADDH) renewed its call for an investigation after allegations Hirak activists were tortured in detention.
During the trial in February of Hirak activist Walid Nekkiche, the 25-year-old student said he was "sexually, physically and verbally assaulted" by members of the security services while in police custody.
The public prosecutor later announced an investigation.