JERUSALEM, Aug 6 (Reuters) - Israel has conducted a systematic policy of prisoner abuse and torture since the start of the Gaza war, subjecting Palestinian detainees to acts ranging from arbitrary violence to sexual abuse, Israeli rights group B'Tselem said in a report. It said the report, issued on Monday, was based on interviews with 55 Palestinians from Gaza, the West Bank and Israel detained in Israeli prisons since Hamas' Oct. 7 attack on Israel that set off the war, most of them without being tried.