The UN has accused China of "serious human rights violations" in a long-awaited report into allegations of abuse in Xinjiang province.
The report assesses claims of abuse against Uyghur Muslims and other ethnic minorities, which China denies.
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-west China.
China had urged the UN not to release the report - with Beijing calling it a "farce" arranged by Western powers, the BBC reported.
Investigators said they uncovered "credible evidence" of torture possibly amounting to "crimes against humanity".
The UN 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".
There are about 12 million Uyghurs, mostly Muslim, living in Xinjiang. The UN said non-Muslim members may have also been affected by the issues in the report, the BBC added.
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. Its delegation to the UN human rights council in Geneva rejected the findings of the report, which it said "smeared and slandered China" and interfered in the country's internal affairs.
"This so-called 'assessment' is a politicized document that ignores the facts, and fully exposes the intention of the US, Western countries and anti-China forces to use human rights as a political tool," BBC quoted a lengthy Chinese statement as saying.
The report was released on UN High Commissioner for Human Rights Michelle Bachelet's final day on the job after four years since 2018.
Her term has been dominated by the accusations of abuse against the Uyghurs.
Bachelet's office indicated that an investigation into allegations of genocide in Xinjiang was under way over a year ago, the BBC said.
But publication was delayed several times, leading to accusations by some Western human rights groups that Beijing was urging her to bury damaging findings in the report.
And even in the final hours before the report was published, China has been putting pressure on Bachelet not to release it, BBC added.
In a news conference last Thursday, she admitted that she was under "tremendous pressure to publish or not to publish" the report.
But she defended the delay, arguing that seeking dialogue with Beijing over the report did not mean she was "turning a blind eye" to its contents.
Sophie Richardson, China director at Human Rights Watch, said the report's findings showed "why the Chinese government fought tooth and nail to prevent the publication" of the report.
"The United Nations Human Rights Council should use the report to initiate a comprehensive investigation into the Chinese government's crimes against humanity targeting the Uyghurs and others - and hold those responsible to account," she added.