Twitter said it removed 936 accounts it said were being used to “sow political discord in Hong Kong”.
The network said the accounts originated in mainland China and were part of a coordinated attempt to undermine the “legitimacy and political positions of the protest movement”.
Facebook said it had, after being tipped off by Twitter, removed "seven Pages, three Groups and five Facebook accounts.”
"They frequently posted about local political news and issues including topics like the ongoing protests in Hong Kong,” said Nathaniel Gleicher, Facebook's head of cybersecurity policy.
"Although the people behind this activity attempted to conceal their identities, our investigation found links to individuals associated with the Chinese government."
In addition to the 936 specific accounts, Twitter said as many as 200,000 other accounts, designed to amplify the misinformation, were “proactively” suspended before becoming "substantially active”.
"Based on our intensive investigations,” the firm said in a statement, “we have reliable evidence to support that this is a coordinated state-backed operation.
"Specifically, we identified large clusters of accounts behaving in a coordinated manner to amplify messages related to the Hong Kong protests.”
It added: "We will continue to be vigilant, learning from this network and proactively enforcing our policies to serve the public conversation.”
The move came after Twitter was intensely criticised at the weekend for allowing China’s Xinhua news agency to buy sponsored posts on the network. Twitter said on Monday it would no longer allow such ads.
"Going forward, we will not accept advertising from state-controlled news media entities,” the company said. "Any affected accounts will be free to continue to use Twitter to engage in public conversation, just not our advertising products.”
Twitter said the new policy did not, however, apply to "to taxpayer-funded entities, including independent public broadcasters”.