LONDON—A “people’s tribunal” set up to assess whether China’s alleged rights abuses against the Uyghur people constitute genocide opened in London on Friday, with witnesses alleging that inmates at detention camps for Uyghurs were routinely humiliated, tortured, and abused. Chairman Geoffrey Nice said more than three dozen witnesses would make “grave” allegations against Chinese authorities during four days of hearings. The tribunal, made up of lawyers, academics, and businesspeople, does not have UK government backing or any powers to sanction or punish China. But organizers hope the process of publicly laying out evidence will compel international action to tackle alleged abuses against the Uyghurs, a largely Muslim ethnic group. Nice, a British barrister who led the prosecution of ex-Serbian President Slobodan Milosevic and has worked with the International Criminal Court, said the forum would create “a permanent body of evidence and a record, if found, of crimes perpetrated.” Funded by …