Hong Kong police on Monday said they had arrested five people over hacking allegations, days after the online "hacktivist" group Anonymous declared a cyber-war against the southern Chinese city.

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 on Monday.

The demonstrators are demanding free and open elections to select the former British colony's next leader in 2017. China's Communist authorities insist only pre-approved candidates will be able to run, a system critics dismiss as "fake democracy".

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 protestors, 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.