As well as the UDHR, we also have international human rights treaties such as the International Covenant on Civil and Political Rights (ICCPR). As a treaty, it is binding on those countries which have ratified or acceded to it (signature alone does not legally bind one to a treaty, it merely expresses an intention to ratify it in the future.) On that point, it is worth noting that PRC signed the ICCPR in 1998, but as yet has not ratified it, so is not legally bound to it. (Previously, in 1967, ROC signed it, but never ratified it either; PRC's position is that ROC's 1967 signature was legally void, hence their own signature in 1998.)
(Technically speaking, an unratified signatory, while not bound to the terms of the treaty itself, is obliged under Article 18 of the Vienna Convention on the Law of Treaties (VCLT) to "refrain from acts which would defeat the object and purpose of a treaty" – but it is difficult to see how that article could be applicable to the case of China and the ICCPR.)