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 found "credible evidence" of torture possibly amounting to "crimes against humanity".
The report was released on Michelle Bachelet's final day on the job after four years as the UN's high commissioner for human rights. Her term has been dominated by the accusations of abuse against the Uyghurs.
Her team's report accused China of using vague national security laws to clamp down on the rights of minorities and establishing "systems of arbitrary detention".
It 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-west China.
The World Uyghur Congress, an umbrella group representing about 60 organisations, welcomed the report and urged a swift international response.
"This is a game-changer for the international response to the Uyghur crisis," Uyghur Human Rights Project Executive Director Omer Kanat said.
"Despite the Chinese government's strenuous denials, the UN has now officially recognised that horrific crimes are occurring."
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 US and lawmakers in several other countries have previously denounced China's actions in Xinjiang as a genocide, but the UN stopped short of making the accusation.
Beijing - which saw the report in advance - denies allegations of abuse and argued that the camps are a tool to fight terrorism.
China has always insisted that Uyghur militants are waging a violent campaign for an independent state by plotting bombings, sabotage and civic unrest, but it is accused of exaggerating the threat in order to justify repression of the Uyghurs.
Its delegation to the UN human rights council in Geneva on Thursday 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 politicised 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," it said in a lengthy statement.
Will the UN report lead to change?
Tessa Wong, Asia Digital Reporter
The ball is now in the global community's court. Already, Uyghur rights activists are calling for a commission of inquiry to be set up, and asking businesses around the world to cut all ties with anyone abetting the Chinese government in its handling of the Uyghurs.
Whether increased international pressure will result in concrete change is debatable. Beijing has doubled down on its stance, denying that atrocities have taken place and insisting it is a victim of a Western-led smear campaign. It has even claimed that Xinjiang is now socially stable and economically developed, calling it "the greatest human rights achievement".
The UN report is no doubt embarrassing, further chipping away at China's insistence that it is a responsible member of the international community.
But this investigation is just the latest in a long series of damning reports that Beijing has summarily dismissed.
Domestic pressure is also unlikely. The issue of Uyghur human rights abuses has never been a top concern for many Chinese, mostly because it has long been a taboo topic and heavily censored - as of Thursday afternoon, the UN report had yet to be mentioned in Chinese mainstream media or social media platforms.
The fate of the Uyghurs depends on when the Chinese government decides it has fully vanquished what it sees as the threat of terrorism and radicalism in the community.
It is anyone's guess when it will reach that conclusion - if ever.
Ms Bachelet's office indicated that an investigation into allegations of genocide in Xinjiang was under way over a year ago.
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.
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.
And Amnesty International's Secretary General Agnès Callamard condemned "the inexcusable delay" in publishing the findings.
"There must be accountability for the Chinese government's crimes against humanity, including through the identification and eventual prosecution of those individuals suspected of responsibility," Ms Callamard said.
Earlier this year, the BBC obtained leaked files which revealed an organised system of mass rape, sexual abuse and torture of Uyghur Muslims at a network of camps.
The Xinjiang Police Files were passed to the BBC and revealed a targeting of the community on orders leading all the way up to Chinese leader, Xi Jinping.
And in 2020, then UK Foreign Secretary Dominic Raab accused China of "gross and egregious" human rights abuses against its Muslim population after a video emerged appearing to show Uyghurs being blindfolded and led to trains.
The footage provoked international outcry, but Liu Xiaoming, then Chinese ambassador to the UK, insisted that there were "no such concentration camps in Xinjiang" while appearing on the BBC's Andrew Marr show.
Credit: The BBC