By Tuesday afternoon, the hashtag “Choi Siwon” was the number one topic on Weibo, with 260 million views.
A Choi fan group in China announced on Weibo it was closing as a result of the fracas.
“No one and nothing can shake our own position about patriotism,” the group posted Monday.
The Chinese state-run Global Times said Choi had “liked” a post that “glamorized Hong Kong rioters”.
Other online users also expressed their anger.
“(I) will not forgive you, because my country is more important,” wrote one.
Fans also accused Choi of not being sincere in his apology, and criticised him for posting it only to Weibo and not to other platforms.
Choi had already faced controversy in South Korea when a renowned restaurateur died after being attacked by his family’s pet dog in 2017.
A number of international brands and celebrities have found themselves facing Chinese anger after being perceived to adopt a stance on the pro-democracy protests that have rocked Hong Kong for months.
One of the most high-profile rows saw popular Houston Rockets basketball games pulled from the air by state broadcasters after the Rockets general manager posted a later-deleted tweet supporting the protesters.