In a Hong Kong tower block prostitutes in one-room brothels sell themselves for as little as US$40 a time and are often exploited by triad gangsters and abusive clients.

But an increasing number of sex workers, particularly migrants from mainland China, are complaining about the behaviour of the police supposed to enforce the law, with some officers accused of helping themselves to free sexual services.

Hundreds, if not thousands, of sex workers ply their trade in one-room brothels, massage parlours, saunas and bars throughout the southern Chinese city, where prostitution is legal but soliciting is not.

In an anonymous 20-storey tower block close to a popular shopping district, men of all ages take the lift to the top of the building, avoiding eye contact with each other as they circle down the dingy stairwells, ringing at doors where signs say either 'welcome' or 'wait'.

Muffled noises betray what is happening inside.

In one of the rooms, a 25-year-old prostitute sits on the edge of the bed, wearing only black knickers and a white t-shirt sporting a picture of a cartoon bird.

With its pink duvet, makeup kits, soft toys and television, the messy space could belong to a typical teenage girl.

But the woman laughed uneasily and held out her wrist to show a network of cuts, some fresh, some faded, all self-inflicted.

"This job is no good. In my heart I am unhappy, but my face must be happy," she said.

She is controlled by a member of one of the city's notorious triad gangs. Forced to work as a prostitute, she has to pay the gang member HK$2,000 (US$260) a day since an ex-boyfriend amassed huge gambling debts six years ago, using her as a guarantor.

Every day she sees five or six clients, or 'fans' as she prefers to call them, earning HK$500 for 40 minutes, paying the HK$2,000 to her boss and keeping whatever is left.

She says she cannot escape or call the police because the triads will take revenge on her family, who do not know of her work.

"The girls here are all controlled," she said. Most are mainly Chinese and so more vulnerable to police harassment than those from Hong Kong, she added.

"The police are afraid of Hong Kong girls because we can complain but they are not afraid of China girls. If the China girls complain they can arrest them," she said.

-- "I never thought Hong Kong had police like this" --

It is against Hong Kong law for anyone to live off the earnings of a prostitute, and police say they are targeting the organised criminals controlling the women.

But Zi Teng, a group campaigning for sex workers' rights, collected 203 complaints from sex and massage workers against police in the first half of the year, more than double the figure for the previous six months.

The allegations included 32 accusations of officers taking free sexual services and eight of indecent assault, along with arbitrary arrests.

There is no way of verifying the allegations and police say they attach "great importance to the proper conduct" of officers.

Nonetheless Zi Teng project officer Betty Shao said controversial police rules, which allow undercover officers to receive sexual services including masturbation as part of an investigation, reflect the city's misguided policies when it comes to the sex industry.

"Sex work is like any other type of work and sex workers should have the same rights," she said.

"These three types of person -- the police, bad customers and bad bosses -- they're the ones who are acting illegally, but the law cheats the migrant sex worker first."

Hong Kong has seen a huge influx from mainland China since the handover from British control in 1997, with the number of visitors rising 15-fold to almost 28 million in 2011.

Police figures show arrests related to offences involving sex workers from the mainland quadrupled in the same period to 3,752.

These migrants are often poorly educated, with a limited understanding of the language and law. Zi Teng says they do not report crimes to police because many are working illegally and fear being arrested themselves.

Some work in massage parlours, often fronts for sex industry premises.

Establishments need a licence for full body massage and Zi Teng says there are cases in which undercover officers have made several visits -- receiving sex services at the taxpayers' expense -- before making an arrest.

JoJo, a 35-year-old from southwestern China, was charged with managing an unlicensed massage parlour after police raided the establishment where she was working in March. She was found guilty but is appealing the court's decision.

She insists she was giving only a foot massage, and that during the raid an officer took her to a side room and fondled her breast.

When she was bailed at the police station, she refused to leave until somebody dealt with her complaint. At this point, she alleges, she was forced to the ground and kicked in the leg and head.

"I thought these kind of police only existed in mainland China. I never thought that Hong Kong, such an international city, also has police like this," she said, wiping away tears.

Elsewhere in Hong Kong, in Wanchai's red light district, Filipinos and Thais work on six-month "entertainment" visas, making commission from drinks bought for them by foreigners, before going back to their hotel rooms.

A female manager -- or "mama-san" -- at one venue said the women charge HK$1,800 for sex, keeping half, and giving half to the Chinese bar owners.

"Of course, the police know what's going on," she said, referring to this type of organised prostitution. But she added that officers turn a blind eye, in part because the women have work visas.

-- Legal and ethical issues --

Hong Kong police declined to be interviewed but said in a statement that their 'major objective' was "to combat illegal vice activities including harbouring or exercising control over a woman for the purposes of prostitution".

"Police have clear and strict guidelines to regulate undercover operations," the statement said. The operations are monitored by senior officers "to ensure that the limited sex services accepted during the operation are genuinely necessary".

The force faces embarrassment after two sergeants and a constable were arrested by Hong Kong's Independent Commission Against Corruption, accused of 'accepting advantages' in return for protecting vice and gambling activities.

Simon Young, the director of the Centre for Comparative and Public Law at Hong Kong University, said that given the increasing mobility of workers from China in the past 15 years, the city should review policy towards its sex workers.

One area to examine is the long-standing force rules about accepting sexual services as part of an investigation.

"That raises legal issues and more importantly ethical issues," he says.

"We hear stories of officers going once, twice, multiple times, in the name of gathering evidence. One wonders whether they are simply abusing the process."

Back in the one-room brothel the 25-year-old has two years left before her debts are paid. In the meantime she relies on her fellow sex workers for support.

"Police and the public are the same," she said. "They think it's normal if something bad happens to us."