Amnesty International is demanding China account for as many as one million ethnic Muslims the group says have been arbitrarily detained in the remote western Xinjiang region.
In a report released Monday, the human rights watchdog says the region's Uighurs, Kazakhs and other predominantly Muslim ethnic groups have been the targets of "an intensifying government campaign of mass internment, intrusive surveillance, political indoctrination, and forced cultural assimilation."
Beijing began a campaign in April 2017 to arrest Uighurs accused of holding extremist sentiments and send them to re-education camps.
Amnesty says "open or even private displays of religious or cultural affiliation," including wearing a beard, veil or headscarf, regular prayer or possessing written material about Islam or Uighur culture can be considered extremist.
"The Chinese government must not be allowed to continue this vicious campaign against ethnic minorities" in Xinjiang, said Nicholas Bequelin, Amnesty's East Asia director.
"Hundreds of thousands of families have been torn apart by this massive crackdown," he continued. "They are desperate to know what has happened to their loved ones and it is time the Chinese authorities give them answers.
Beijing says Xinjiang is facing a serious threat from Islamist militants who plot attacks and stir up tensions among the mostly-Muslim Uighur minority against the ethnic Han Chinese majority.