His Majesty's Prison Belmarsh is a Category A men's prison in Thamesmead, southeast London, England. The prison is used for high-profile cases, particularly those concerning national security. Within the grounds is the High Security Unit (HSU), which consists of 48 single cells. It is run by His Majesty's Prison Service. The prison has been called "Britain's Guantanamo Bay" due to the long-term detention of terrorism suspects without charge. Considered the toughest prison in the UK, Belmarsh is also notoriously known as "Hellmarsh" due to the high number of physical and authority abuses reported by both the prison's inmates, and by human rights activists.