Enzai: Falsely Accused [OVA]
The protagonist is Guys, a young boy from a poor family, who gets caught for stealing candy from a Paris store. And of course he will be investigated and punished.
However, after being railroaded by a city detective named Guildias, Guys finds himself accused, convicted, and sentenced to life imprisonment for the murder of a man he never met.
From that point on, most Enzai takes place inside a dark, claustrophobic, dirty prison, in there, Guys experiences humiliation and torture of various kinds, much of it involving sexual acts such as rape and forcible sodomy.