UN High Commissioner for Human Rights Michelle Bachelet, who has faced criticism from some diplomats and rights groups for being too soft on China, released the report just minutes before her four-year term ended. She visited China in May.
The UN Human Rights Office said in its 48-page report that "serious human rights violations have been committed" in Xinjiang "in the context of the government's application of counter-terrorism and counter-'extremism' strategies".
"The extent of arbitrary and discriminatory detention of members of Uighur and other predominantly Muslim groups ... may constitute international crimes, in particular crimes against humanity," the UN office said.
It recommended the Chinese government take prompt steps to release all those detained in training centres, prisons or detention facilities.
"There are credible indications of violations of reproductive rights through the coercive enforcement of family planning policies since 2017," the office said.
It added that a lack of government data "makes it difficult to draw conclusions on the full extent of current enforcement of these policies and associated violations of reproductive rights."
Rights groups accuse Beijing of abuses against Uighurs, a mainly Muslim ethnic minority that numbers around 10 million in the western region of Xinjiang, including the mass use of forced labour in internment camps. The United States has accused China of genocide.
China has vigorously denied any abuses in Xinjiang and issued a 131-page response to the UN report.
Chinese foreign ministry spokesperson Wang Wenbin described the report as "completely illegal and void".
"This proves once again that the OHCHR has become a thug and accomplice of the US and the West," he said during a regular daily briefing on Thursday in Beijing, where he was asked repeatedly about the report.
China's ambassador to the United Nations in New York, Zhang Jun, said Beijing had repeatedly voiced opposition to it.
"We all know, so well, that the so-called Xinjiang issue is a completely fabricated lie out of political motivations and its purpose definitely is to undermine China's stability and to obstruct China's development," Zhang told reporters on Wednesday.
Bachelet said she wanted to deal with input from the Chinese government last week in the spirit of constructive dialogue.
"To be perfectly honest, the politicisation of these serious human rights issues by some states did not help," she added.
Dilxat Raxit of the World Uighur Congress, an international organisation of exiled Uighur groups, said the report confirmed "solid evidence of atrocities" against Uighurs, but wished it had gone further.
"I regret that the UN Office of the High Commissioner for Human Rights did not characterise these extreme atrocities in China as genocide," he told Reuters in an email.
Reuters reported last month that China had asked Bachelet to bury the report, according to a Chinese letter that was confirmed by diplomats.
Bachelet confirmed last week having received the letter which she said was signed by about 40 other states, adding her office would not respond to such pressure.
Bachelet, 70, plans to return to Chile to retire.
"Frankly to issue the report as she's walking out the door minimises the report," Kenneth Roth at Human Rights Watch told Reuters, before it was released.
Still, Human Rights Watch described the report as groundbreaking.
"Victims and their families whom the Chinese government has long vilified have at long last seen their persecution recognised, and can now look to the UN and its member states for action to hold those responsible accountable," said John Fisher, its global advocacy deputy director.
