GENEVA — United Nations human rights experts expressed alarm on Friday over what they said were many credible reports that China had detained a million or more ethnic Uighurs in the western region of Xinjiang and forced as many as two million to submit to re-education and indoctrination.

In the name of combating religious extremism, China had turned Xinjiang into “something resembling a massive internment camp, shrouded in secrecy, a sort of no-rights zone,” Gay McDougall, a member of the United Nations Committee on the Elimination of Racial Discrimination, said in the opening session of a two-day review of China’s policies in Geneva.

Accounts from the region pointed to Muslims “being treated as enemies of the state solely on the basis of their ethno-religious identity,” Ms. McDougall said, citing reports from activists and scholars that many had disappeared and that even the most commonplace religious practices had become grounds for punishment.

Raising questions about the fate of Uighur students who had returned to Xinjiang from overseas, Ms. McDougall said that more than a hundred had disappeared, some had been detained and others had died in detention.