Tragic footage of police mistreating a helpless man in Iran's second-largest city, Mashhad, has triggered a series of angry protests on the Persian social media.

Mehdi Akhlaqi, the military prosecutor of Khorasan Razavi province in northeast Iran, announced on Saturday that a legal case had been filed to investigate the security forces' role in the death of a young man in its custody.

The video, widely circulated on social media, shows a police officer using pepper spray and taser on the face and body of a man handcuffed and tied to a metal bar.

"A legal case has been filed in the Khorasan Razavi military prosecutor's office, and the family of the deceased has also filed a complaint with the office," Khorasan Razavi's military prosecutor, Mehdi Akhlaqi, said on Saturday.

"The Deputy Military Prosecutor of Khorasan Razavi visited the morgue, and after the autopsy, ordered a lung biopsy, and an investigation into the effects of pepper spray on the death of the deceased," Akhlaqi told reporters.

He also asserted that the case had been referred to the Investigation Branch of the Khorasan Razavi Military Court.

The BBC's Persian Service has identified the victim as Mehrdad Sepehri.

The victim's family insist that he "died of suffocation with pepper spray, while inside a police car."

The commander of the police force in Khorasan Razavi province, Mohammad Kazem Taqavi, also said, following the release of the video on "hostile networks" showing the death of the young man amid allegations of "pepper spray poisoning," the officer involved in the case has been summoned, and an internal investigation was underway.

The release of a video of a helpless girl being beaten in Abadan, southwest Iran, by two women and an oil company's home security officer has also provoked widespread protests in recent days on social media, with authorities in the oil-rich province of Khuzestan promising to investigate the case.