A trial challenging Japan's lengthy and grueling detention of criminal suspects opened in Tokyo on Wednesday, with plaintiffs decrying "subhuman" treatment they say ignores the presumption of innocence.

Campaigners argue that lengthy pretrial detention is meted out too easily in Japan, especially if suspects remain silent or refuse to confess.

The lawsuit is challenging the constitutionality of the system of "hostage justice" in which confessions become a de facto condition for their release.