Hong Kong -- Pushing back against China's growing disinformation campaign targeting Hong Kong's sustained pro-democracy protests, U.S. social media tech giants Twitter and Facebook announced the suspension and removal of hundreds of thousands of accounts on Tuesday. Many of the accounts, which were accused of attempting to sow political discord in Hong Kong, were based in mainland China, which officially blocks access to both social media sites.

"We are disclosing a significant state-backed information operation focused on the situation in Hong Kong, specifically the protest movement and their calls for political change," Twitter said in a press release. The company said 936 of the "most active" accounts were based in China. A bigger network of 200,000 "more spammy" accounts was also suspended.

Twitter said it suspended the accounts because they violated its policies, which prohibit spam, coordinated activity and fake accounts.

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Also on Tuesday, Facebook announced it had removed "seven Pages, three Groups and five Facebook accounts involved in coordinated, inauthentic behavior as part of a small network that originated in China and focused on Hong Kong." The social media site said about 15,500 accounts had been following at least one or more of the Pages in question, and about 2,200 accounts had joined at least one of the removed Groups.

Links to the Chinese government

Both companies said the accounts they removed presented themselves as independent but were actually linked to the Chinese state. A tactic of state-run disinformation campaigns is to create the impression, through fake or misleading accounts, that public opinion supports their agenda.

One tweet that was removed, posted by a writer with ties to Beijing, said:

"On the night of 1st July, all people in HK witnessed, from TV or phones, this scene of violence at the Legco attack. The rioters severely vandalised. Do they still think they can win HK people's heart?"

On Facebook, two removed posts compared Hong Kong's pro-democracy protestors with terrorist fighters from ISIS. Another said they were like cockroaches that need to be killed.

"Twitter did a good job to make it serious and target it accurately, which is imaginably a big effort," Isaac Mao, one of Greater China's most high-profile internet and social media researchers, told CBS News. As for Facebook's efforts, he said: "It's a joke, honestly," adding that one of China's biggest internet troll groups, known as Diba, "was still up running there without being touched."

Great Firewall of China

Many Western social and news media companies, including Twitter, Facebook, Instagram, Gmail and Slack, can only be accessed in mainland China via a VPN -- a virtual private network that lets users circumvent the so-called Great Firewall of China to connect to the outside world.

"Chinese media use foreign social media to communicate with people around the world, to introduce them to Chinese policies and tell China's story," Geng Shuang, China's Foreign Ministry spokesman told journalists in Beijing.

"I don't know why certain companies or peoples' reactions are so strong. I don't know if this hits some of their weaknesses," he said.

Chinese state media and related nationalistic sites accused Twitter and Facebook of having a double standard.

"Chinese netizens are only expressing their voice of safeguarding national unity" but western media are "magnifying or overstating the opposition's protests, but understating or even ignoring violence," Guanchazhe, a nationalistic platform in China, said in an online post.

"If China's disinformation campaign had begun earlier in this protest movement, then Hong Kong people might have been influenced, but people have immunity now," Bonnie Cheung, one of the co-leaders of Hong Kong's Civil Humans Right Front (CHRF) that organized the four Sunday marches, which have taken place in the city since June 9th, said.

"As for international audiences, they need to be cautious about what they read online," she continued.

Hong Kong is one of Asia's most international cities and was, until recently, regarded as a stable global finance capital. It has been rocked by more than ten weeks of protests, violence and chaos: What started out as public frustration against a contentious extradition bill has exploded into anger and calls for greater democratic reform, the resignation of the city's chief executive, Carrie Lam, and an investigation into police brutality. Millions of people have taken to the streets since the first million-person march on June 9th.

On Sunday, hundreds of thousands of Hong Kongers peacefully marched across the city, defying a government ban and rainy weather. Organizers said more than 1.7 million people participated in the march, though Hong Kong's police force put that number at 128,000. It was the first weekend in the protest movement's 11 weeks in which Hong Kong's police force fired no tear gas at protestors.

After Sunday's non-violent protest, Hong Kong Chief Executive Carrie Lam said her government would "immediately start the work to establish a platform for dialog."

Lam's approval ratings have plunged to historic lows – 20% according to the most recent poll by the Hong Kong Public Opinion Research Institute – since the U.K. returned the territory to Chinese rule in 1997.