However, the law does not expressly state whether a 'public place' also covers internet forums and smartphone apps - the new front line of the sex trade, making arrest and convictions for compensated dating difficult.

Superintendent Brian Lowcock of the Kowloon West regional crime unit said that once arrested, young women on the dates normally denied they provided sexual services. 'They only say they met a friend on the internet, but were sexually attacked when they hung out,' he said.

Police have requested legal advice from the Department of Justice on their powers to tackle those seeking potential dates in the cyberworld.

Lowcock said it was important to clarify whether internet forums were public places under the law because it would help police pursue those seeking sex services and those offering them online.

He also said it would be helpful if there were a legal precedent in Hong Kong.

Police have also mounted an operation online to keep an eye on internet forums and smartphone apps that seem to be a front for this area of the sex trade.

Lowcock said police were aware of some smartphone apps that allowed users to make friends with nearby users using location-based technology. But so far they did not point to any compensated-dating activity. He said police were also conducting a legal study to see whether such platforms could be defined as public places so that the same offence could apply.

Detective Chief Inspector Alan Chung, also of the Kowloon West regional crime unit, said that after police put up warnings on some popular forums, people started to use less obvious terms to look for dating partners.

'Some only used [compensated dating] jargon. Besides the legal difficulties, another problem is that we need to prove that the person [we arrest] is really the one who posted the things online ... and solicits others for immoral behaviour,' he said.

The warnings seem to have had little effect. On one popular compensated-dating website, hkbigman.net, the police warnings were clear but so were the photos and prices of teenage girls offering services.

It has been a year since the police set up a team to combat compensated dating and in that time 28 people have been arrested in 25 cases. Most were sentenced to four to 16 months, some only to community service.

The youngest people arrested were a boy and a girl, both 13. The boy acted as a pimp while the girl provided the service.

Similar operations were conducted in 2008, ending in 15 arrests, and in 2009, with 19 people arrested. Those arrested ranged in age from 13 to 27 and included men offering dates with homosexual partners.

Chung said most of the cases in the past year involved girls working on their own or relying on introduction from agents or classmates, earning HK$400 to HK$3,000 each time.

Some of the cases though were linked to two syndicates, which each managed seven or eight girls.

Lowcock said education and prevention would be more effective in eradicating compensated dating than amending the law. That is why the police were working closely with schools and community groups to tackle the problem.

He warned that compensated dating could be dangerous, citing the example of 16-year-old Wong Ka-mui, who was murdered in 2008 by her boyfriend-for-a-day.