Hong Kong’s protests have disrupted Yang Yang’s family life. Though the 29-year-old lives in mainland China, he was inspired by the demonstrations to write a song about freedom and upload it to the internet. When censors deleted it, he complained to his family.

They weren’t sympathetic. “How can you support Hong Kong separatists?” they asked. “How can you be anti-China?” His mother threatened to disown him. Before Mr. Yang left on a trip to Japan in August, his father said he hoped his son would die there.

Hong Kong’s protests have inflamed tensions in the semiautonomous Chinese city, but passions in the mainland have been just as heated — and, seemingly, almost exclusively against the demonstrators.

A pro-protest tweet by a Houston Rockets executive, Daryl Morey, ignited a firestorm of anger against the N.B.A., demonstrating the depth of feeling. Joe Tsai, the only N.B.A. owner of Chinese descent, said all of China — yes, more than one billion people — felt the same way.