Weibo said in a statement Friday it had begun a "clean-up campaign" to remove "illegal" content, including "manga and videos with pornographic implications, promoting violence or (related to) homosexuality".
It is the latest sign in a crackdown by the ruling Communist Party to purge the Internet of any content deviating from its "core values of socialism" while stifling criticism of social norms and established policies.
The three-month campaign will also tackle "violent video games, like 'Grand Theft Auto'," Weibo said on the official account of its administrators.
The popular Twitter-like platform, which boasts 400 million active monthly users, said it was implementing China's new cybersecurity law and had already removed some 56,240 items by Friday evening.
The announcement provoked a flood of reaction from stunned or outraged Chinese Internet users, with protesters rallying behind the hashtag "I am gay".
By midday Saturday, it had been used by some 170,l000 Weibo users, before it was apparently banned by the platform.
"There can be no homosexuality under socialism? It is unbelievable that China progresses economically and militarily but returns to the feudal era in terms of ideas," one angry commenter said.
"How is it that public opinion has narrowed so much in the last two years?" said another.
China only decriminalised homosexuality in 1997, but conservative attitudes remain widespread.
"It's simply discriminatory! Many mangas removed were not pornographic," observed a third.
The large online community of "funu" ("deviant girls"), heterosexual women who are avid fans of male gay romances and share comics or stories, was particularly critical.
Many messages protesting at the content crackdown were deleted.
Authorities closely monitor the Internet to purge any content deemed sensitive, such as political criticism or pornography, and require websites to have their own censors.
China has seen a tightening wave of censorship under the leadership of President Xi Jinping, who advocates a stronger promotion of socialist ideology in society.
One of its latest victims was Toutiao, one of China's most popular news aggregator apps, which was punished this week for allowing users to share ribald jokes or videos and has promised to increase its censorship staff to 10,000.
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