A United Nations human rights panel said it had received many credible reports that 1 million ethnic Uighurs in China are held in what resembles a "massive internment camp that is shrouded in secrecy".
Gay McDougall, a member of the UN Committee on the Elimination of Racial Discrimination, cited estimates that 2 million Uighurs and Muslim minorities were forced into "political camps for indoctrination" in the western Xinjiang autonomous region.
"We are deeply concerned at the many numerous and credible reports that we have received that in the name of combating religious extremism and maintaining social stability (China) has changed the Uighur autonomous region into something that resembles a massive internship camp that is shrouded in secrecy, a sort of 'no rights zone'," she said on Friday at the start of a two-day regular review of China's record, including Hong Kong and Macao.
China has said that Xinjiang faces a serious threat from extremist militants and separatists who plot attacks and stir up tensions between the mostly Muslim Uighur minority who call the region home and the ethnic Han Chinese majority.
A Chinese delegation of some 50 officials made no comment on her remarks at the Geneva session that is scheduled to continue on Monday.
The US mission to the United Nations said on Twitter that it was "deeply troubled by reports of an ongoing crackdown on Uighurs and other Muslims in China".
"We call on China to end their counterproductive policies and free all of those who have been arbitrarily detained,” it said.
The allegations came from several sources, including activist group Chinese Human Rights Defenders, which said in a report last month that 21 per cent of all arrests recorded in China in 2017 were in Xinjiang.
Earlier, Yu Jianhua, China's ambassador to the United Nations in Geneva, said it was working towards equality and solidarity among all ethnic groups.
But Ms McDougall said that members of the Uighur community and other Muslims were being treated as "enemies of the state" solely on the basis of their ethno-religious identity.
More than 100 Uighur students who returned to China from countries including Egypt and Turkey had been detained, with some dying in custody, she said.
The review is being held as thousands of Chinese Muslims from the Hui ethnic minority are protesting to prevent the demolition of a recently rebuilt mosque in Weizhou, a town the northern Ningxia region.
Although Islam is one of five officially recognised religions in China, home to about 23 million Muslims, the ruling Communist Party has recently called for the "Sinicisation" of religious practice to bring it in line with "traditional" Chinese values and culture.
The party-affiliated Global Times newspaper said on Saturday that no religion was above the law and urged officials to deal firmly with the protest in Weizhou.
The paper said local officials should act against what it described as an illegal expansion of a religious building.
A supposed government order circulated in the town last week called for the mosque to be demolished by August 10, saying it had been rebuilt without the proper permits.