Pirated video discs are on sale with Chinese subtitles
Authorities believe a piracy ring taped the film when it opened in the United States on 16 November, meaning it could be copied and sold more than three weeks before its cinema release in Hong Kong.
Video CDs (VCDs) of the film have been on sale for 20 Hong Kong dollars (£1.80), and have also appeared in other Chinese cities plus Malaysia and Taiwan.
Counterfeiters selling pirate DVDs in Beijing and elsewhere in China have even added Chinese subtitles, according to reports.
The Hong Kong arrests and seizures came at two premises, officials said.
"Since the film is not yet released in Hong Kong, we believe a piracy syndicate filmed the movie in cinemas in the United States and sent the copy to Hong Kong for duplication," senior inspector of Hong Kong Customs, Lee Hon-wah, said.
Counterfeiters face up to four years in jail and a fine of HK$50,000 (£4,500) per fake item.
The film will not reach the territory's cinemas until 20 December.
In 1999, Hong Kong was removed from US government piracy watch list after US officials decided that the territory had toughened its approach to the problem.
But despite a crackdown, the latest Hollywood movies can sometimes be found illegally on sale before public release.
Customs officials confiscated 880,000 pirated films in the first six months of this year, on top of 1.53 million discs seized in 2000.
Because of attempts to wipe out on the problem, pirate video stalls are sometimes left unmanned - and rely on shoppers' honesty to put money into a shoe box at the door.
Car boot sales
There have also been reports of pirate Harry Potter videos being sold in the UK.
The News Of The World reported that about 10,000 videos were circulating in markets and car-boot sales - but said that technical quality was very low.
The Harry Potter film has been a big hit on both sides of the Atlantic since its release.
Harry Potter and the Sorcerer's Stone, as it is known in the US, took almost $190m (£134m) in the US in 10 days - more than Gladiator earned during its whole run in the country.
Meanwhile in the UK, Harry Potter and the Philosopher's Stone had the most successful opening weekend in UK cinema history, smashing the previous record.