Anonymous on Wednesday threatened online sabotage against the police and government, citing the treatment of pro-democracy protesters who have occupied some of the city's main streets over the past week.
The hacking group made the threat after police unleashed tear gas on crowds.
Police senior superintendent Hui Chun-tak told reporters that five people between the ages of 13 and 39 were arrested for "accessing a computer with criminal or dishonest intent".
The sit-ins for free elections drew tens of thousands at the height of the protest but numbers dwindled today.
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Crowds swelled after police used tear gas on unarmed demonstrators on September 28.
Anonymous said in a video post on YouTube: "If you continue to abuse, harass or harm protesters, we will continue to deface and take every web-based asset of your government off line."
In June, a website where residents could vote in a unofficial poll for constitutional reform came under attack. Organisers blamed Beijing.