Last month, Chinese police invited Wang Gongquan in for a "cup of tea," often a prelude to detention. He had launched a public petition calling for the release of arrested dissident Xu Zhiyong, and the authorities were not amused. But Wang effectively told the police to forget it — he had no time for tea, he was traveling, and he had said everything he wanted to say.

Wang figured he could get away with defying the Chinese government. He was a respected businessman, a multimillionaire who had made a fortune in real estate and in Silicon Valley. He was a social-media sensation, and he had been careful, as he saw it, not to break any laws. But he knew he was taking a chance.

On Friday, the police came for him. More than 20 police officers arrived at Wang's house at 11:30 a.m. with a warrant accusing him of "organizing a mob to disturb public order." He was led off to criminal detention as his wife looked on, she later told fellow activist and columnist Chen Min. The police searched his house for two hours and seized his computer, according to Chen, who had co-written the petition.