The force said the suspect, 34, planned to stage “chopping attacks” targeting officers on specific days in a post on an online forum on July 2.
Police made the arrest in Yau Ma Tei.
Superintendent Tam Wai-shun of the Cyber Security and Technology Crime Bureau said comments made by the suspect were related to the case of an officer being stabbed in Causeway Bay on July 1.
Officers did not rule out further arrests, saying they’d noticed “unlawful” people spreading messages to encourage others to attack the police.
Two people, aged 20 and 26, were detained on Sunday also allegedly for posting anti-police messages on social media on July 2.