The Gasht-e Ershad (Guidance Patrols) are special police units tasked with ensuring the respect of Islamic morals and detaining people who are perceived to be "improperly" dressed.
Under Iranian law, which is based on the country's interpretation of Sharia, women are obliged to cover their hair with a hijab (headscarf) and wear long, loose-fitting clothing to disguise their figures.
Ms Amini allegedly had some hair visible under her headscarf when she was arrested by morality police in Tehran on 13 September. She fell into a coma shortly after collapsing at a detention centre and died three days later in hospital. The force denied reports that officers beat her head with a baton and banged it against one of their vehicles.