Church-run "hell hole" children's homes in Ireland were like Nazi concentration camps, a former resident claimed.
He was in a chain gang polishing floors, bathed in detergent as punishment and sexually assaulted by a woman when he was aged five or six, the witness told the UK's largest ever inquiry into child abuse.
The Sisters of Nazareth order of nuns oversaw St Joseph's Home in Termonbacca in Londonderry and used to bath the children in Jeyes fluid.
The witness claimed: "It was kind of like a Zyklon B gas chamber."
The alleged abuse happened in the 1950s and 60s. He was later transferred to a home in Galway in the Irish Republic owned by the Christian Brothers.
"Essentially a Gulag, a child's prison," he claimed.
He added: "The comparison was two hell holes. Which is better? It is difficult to describe when things are bad, you are on a race to the bottom. Salthill (Galway) was Auschwitz, Termonbacca was Treblinka."