By Robert McMillan and Maria Armental
Twitter Inc. and Facebook Inc. suspended accounts they believe to be part of a Chinese effort to undermine antigovernment protests in Hong Kong, marking the first time the companies have pointed to China as a source of disinformation campaigns, company representatives said on Monday.
Twitter said it took down 936 accounts linked to a "significant state-backed information operation" originating in China.
"These accounts were deliberately and specifically attempting to sow political discord in Hong Kong, including undermining the legitimacy and political positions of the protest movement on the ground," Twitter wrote in a blog post.
Facebook said that following a tip from Twitter, it removed five accounts along with seven pages, with a reach of more than 15,000 accounts, as well as three groups that included more than 2,000 members. "Although the people behind this activity attempted to conceal their identities, our investigation found links to individuals associated with the Chinese government," Facebook said.
Facebook and Twitter are blocked in China by a government-controlled national firewall. That suggests the latest information operations were likely part of a government-sanctioned effort to influence global opinion, said Graham Brookie, director of the Atlantic Council's Digital Forensic Research Lab.
"They are testing what is effective as well as what is accepted or not pushed back on by the international community," he said.
Hong Kong has seen a wave of protests in the past two months as a pro-democracy movement stepped up pressure on the city's Beijing-backed leader, Carrie Lam, to enter into dialogue to resolve the crisis. Protesters and police have squared off in some of the demonstrations.
Mainland China has repeatedly suggested that it could deploy its own security forces, and even its military, to restore order in the semiautonomous Chinese city if local officials can't manage it themselves.
Pressure has been mounting for Facebook and Twitter to counter state-sponsored disinformation campaigns after accusations that Russia meddled in the 2016 U.S. presidential election. The companies said in January that they had removed hundreds of fake accounts from Iran and Venezuela spreading misinformation on their social-media platforms.
Twitter also said on Monday that it would stop running advertising from state media and that it had removed at least one widely circulated advertisement by China's state-run Xinhua News Agency that criticized the protest movement.
Even without advertising, China's state news organizations have considerable reach on Twitter. Xinhua has 12.6 million Twitter followers.
Russian state news outlet RT has 2.9 million followers. It has been blocked from running ads on Twitter since 2017.
Representatives of the Chinese Embassy in Washington, D.C., didn't immediately respond to a request for comment.
China closely monitors and censors internet traffic within its country, including blocking popular western social media sites. The U.S. government and some independent security researchers have accused China of cyberattacks targeting companies, academia and federal agencies. China has denied those allegations.
Facebook said it shared its analysis with law enforcement and industry partners. It said it took down the pages, groups and accounts "based on their behavior, not the content they posted." The company said it was making progress identifying behavior that isn't permitted on its site, but called it an "ongoing challenge."
Twitter said the suspended accounts represented the most active portions of a coordinated state-backed disinformation campaign against the protesters. The accounts, it said, are part of a larger network of about 200,000 accounts that it suspended "before they were substantially active on the service."
Twitter said some of the Chinese accounts were using unblocked internet addresses originating in China, a sign that they were likely operating with the permission of the Chinese government, according to Mr. Brookie.
Screenshots of posts from some suspended accounts showed violent encounters between police and protesters and images of human faces on bodies of cockroaches, according to images posted by Facebook.
Twitter's action is among the most sweeping for the platform. The company said in June that it had taken down 4,779 accounts linked to Iran, the most from any country.
Write to Robert McMillan at Robert.Mcmillan@wsj.com and Maria Armental at firstname.lastname@example.org