He accused Mr. Xi of trying to cover up the coronavirus outbreak in central China. In one of his most daring writings, he urged Mr. Xi to resign, saying, "You're just not smart enough."

Then, over the weekend, Xu Zhiyong, a prominent Chinese legal activist, went silent. The authorities in the southern city of Guangzhou detained him on Saturday, according to Mr. Xu's friends, after he spent nearly two months in hiding. His girlfriend, Li Qiaochu, a social activist, went missing on Sunday, Mr. Xu's friends said.

The activist is the latest critic to be caught up in Mr. Xi's far-reaching efforts to limit dissent in China. The crackdown, which has ensnared scores of activists, lawyers, journalists and intellectuals, is likely to intensify as the ruling Communist Party comes under broad attack for its handling of the coronavirus outbreak, one of its biggest political challenges in years.