HONG KONG — For six days a week, every week, in a crowded office in Kazakhstan’s largest city, Serikzhan Bilash had been ringmaster of one of the most influential efforts detailing China’s internment of Muslim minorities.

Clad in a suit and tie, he would bounce from room to room, consoling distraught relatives of people held in Chinese indoctrination camps in the region of Xinjiang and arranging supplies and housing for those who had recently arrived in Kazakhstan from China. He would also record hours and hours of videos describing the extent of China’s crackdown — speaking in English, Chinese and Kazakh.

This week, however, Mr. Bilash went uncharacteristically quiet.

The police detained him early Sunday in Almaty, the city in southeastern Kazakhstan where his organization is, and flew him to the capital, Astana, where he was placed under house arrest. He is under investigation on accusations of “inciting ethnic hatred,” said his lawyer, Aiman Umarova. A conviction carries a punishment of up to 10 years in prison.

On two short videos sent to his family by the police, Mr. Bilash said that he was physically safe. He urged the members of the group he leads, Atajurt Kazakh Human Rights, to continue their work while he contested the allegations against him.