The UN has accused China of "serious human rights violations" in a long-awaited report into allegations of abuse in Xinjiang province.
China had urged the UN not to release the report - with Beijing calling it a "farce" arranged by Western powers.
The report assesses claims of abuse against Uyghur Muslims and other ethnic minorities, which China denies.
But Investigators said they uncovered "credible evidence" of torture possibly amounting to "crimes against humanity".
They accused China of using vague national security laws to clamp down on the rights of minorities and establishing "systems of arbitrary detention".
The report, which was commissioned by the UN's Office of the High Commissioner on Human Rights, said prisoners had been subjected to "patterns of ill-treatment" which included "incidents of sexual and gender-based violence".
Others, they said, faced forced medical treatment and "discriminatory enforcement of family planning and birth control policies".
The UN recommended that China immediately takes steps to release "all individuals arbitrarily deprived of their liberty" and suggested that some of Beijing's actions could amount to the "commission of international crimes, including crimes against humanity".
While the UN said it could not be sure how many people have been held by the government, human rights groups estimate that more than a million people have been detained at camps in the Xinjiang region, in north-east China.
Several countries have previously described China's actions in Xinjiang as a genocide.
But Beijing - which saw the report in advance - denies allegations of abuse and argued that the camps are a tool to fight terrorism.
