KASHGAR, China—Hundreds of discarded metal bed frames lie jumbled in a grassy lot here behind a recently emptied re-education center for ethnic-minority Muslims in northwest China. Red stickers on them read: “Recognize your mistakes, admit your mistakes, repent.”

Chinese authorities say everyone has completed their studies at such sites—which Beijing describes as vocational schools. Rights groups and Western governments say that about a million people, most of them Uighurs, have been detained in dozens of such centers across...