The head of Iran's prison service has apologised after hackers leaked videos showing the abuse of detainees at Tehran's notorious Evin prison. The security footage showed guards beating prisoners and dragging one along a floor. Mohammad Mehdi Haj-Mohammadi said he took responsibility for the ‘unacceptable behaviour’. Many political prisoners and dual and foreign nationals are held at Evin. BBC's Jiyar Gol says the leaked videos confirm decades of reports of mistreatment and abuse at prisons across Iran. Also, former political prisoners say the footage is nothing compared to what they experienced in detention. They accuse authorities of routinely using sexual, physical and psychological torture - a charge Iran's government denies. The hacked screen showed the message, ‘Evin Prison is a stain of shame on Raisi's black turban and white beard’ - a reference to Iran's new president, who is a hardline cleric and former judiciary chief.