GENEVA — A group of 22 countries has issued a statement urging China to stop the mass detention of ethnic Uighurs and other Muslims in its western Xinjiang region, the first concerted international challenge to a policy China has vigorously defended at the United Nations.

In a letter to the United Nations High Commissioner for Human Rights, Michelle Bachelet, the states told China to uphold its own laws and international obligations, and stop arbitrary incarceration of Uighurs and other Muslim and minority communities, and permit freedom of religion. The letter was delivered Monday and publicly seen on Wednesday.

Britain, France and Germany were among 18 European countries that, joined by Japan, Australia, Canada and New Zealand, drew attention to reports of large-scale arbitrary detentions and asked Ms. Bachelet to keep the Human Rights Council regularly updated on developments.

China experts, drawing on official Chinese documents, satellite imagery and the testimony of families whose relatives have been detained, estimate that China has detained a million or more people in re-education centers and has imposed intrusive surveillance.