Facebook and Twitter say they've removed hundreds of China-linked accounts which were posting content attempting to "sow political discord in Hong Kong".
Amid the ongoing mass protests in the territory, Twitter said it had identified more than 930 accounts that were "deliberately and specifically" posting anti-protest tweets.
It says the campaign included efforts to undermine "the legitimacy and political positions" of the protest movement.
The social network says it has reliable evidence that the campaign was a coordinated state-backed operation.
While Twitter is banned in China, the company says the accounts used VPNs or "specific unblocked IP addresses originating in mainland China" to post the tweets.
Sample of one of the tweets identified as being part of the campaign. Image: TwitterTwitter said: "We are committed to understanding and combating how bad-faith actors use our services".
Chinese government links
Meanwhile, Facebook said it had removed seven Pages, three Groups and five Facebook accounts after Twitter alerted them to the issues.
According to Facebook, the content included pages posing as news organisations.
Similarly to Twitter, Facebook confirmed it had found evidence linking the posts to "individuals associated with the Chinese government".
The firm explained: "Although the people behind this activity attempted to conceal their identities, our investigation found links to individuals associated with the Chinese government.
"We will continue monitoring and will take action if we find additional violations. We’ve shared our analysis with law enforcement and industry partners."
It comes amid the continuing anti-government protests in Hong Kong, which began more than two months ago over a controversial extradition bill.
Activists' demands include the complete withdrawal of the now-suspended bill, and for an investigation to be conducted into the police handling of the protests.
While recent weeks have seen clashes between demonstrators and police, hundreds of thousands of people gathered peacefully this weekend in the area in and around a city park - with some estimates putting the crowd size at 1.7 million.
Chief executive Carrie Lam said she hopes the peaceful protest marks an opportunity to open a fresh "platform of dialogue" and the beginning of "society returning to calm".
