Other allegations against officers included ­forcing women to carry out free sexual services (11 cases), provide free massage services before arresting them (17), making verbal threats and insults (57), carrying out indecent assaults (three) and physically assaulting them (three).

Meanwhile, clients were accused of 125 abuses, Zi Teng said. Among them were theft (56 cases), removing a condom during sex (seven), denial of payment (19), fraud (six) and one of rape.

The latest figures were released to coincide with the 14th International Day to End Violence Against Sex Workers on December 17.

Zi Teng volunteer Wing Pang said the figures were a snapshot of the increasing abuse and exploitation facing prostitutes.

“Some of them may be scared, but some of them feel it is normal, they have become numb to [abuse],” she said. “We do not want to see this increase again.”

Police declined to comment on the rise in complaints, but said the force was “committed to combating illegal prostitution activities”.

Prostitution is legal in Hong Kong if it involves a monetary transaction between a sex worker and a client, but brothels are illegal.

Arrests for unlawful employment involving sex work have been steadily rising since 2011; 4,589 ­people were arrested last year, up from 4,133 in 2014, according to government figures. Arrests for 2013, 2012, and 2011 were 3,829, 3,619 and 3,939 respectively. Although offences included procuring or ­controlling prostitutes and keeping vice establishments, they largely involved alleged overstayers or those entering the city illegally being involved in prostitution. If convicted, offenders may be banned from visiting Hong Kong for two years.

Zi Teng believes the increase in reported arbitrary arrests and licence checks reflects the political pressure on the government and police to appear to be “cleaning up” areas known for prostitution.

Pang said cases of arbitrary arrest generally ­involved those without ID cards, who were suspected of entering the city to undertake sex work.

She said there was evidence women were suffering increasingly violent attacks towards the end of the year because clients needed more money owing to Christmas and Lunar New Year falling close ­together.

On December 1, a sex worker claimed a client tied her up with plastic straps at knifepoint before raping her in a one-woman brothel in Tsuen Wan, and making off with HK$1,600 in cash and her mobile phone worth HK$5,500.

“Clients may also force the sex worker to lower the price,” Pang said.

Outreach programmes run by Zi Teng suggested there was also a rise in the number of sex workers this year. It estimates there were 20,000 operating both part time and full time within any given month.

Puja Kapai, director of the Centre for Comparative and Public Law at the University of Hong Kong, said the latest figures suggested workers faced ­rising levels of abuse, but emphasised that they may also reflect an increased awareness to report violence.

“It is fair to say the situation looks like it is getting worse,” she said. “But I would want it to be made clear whether this was linked to the increasing visibility of gender-based reporting about violence.

“It may have something to do with more Legislative Council discussions about this issue as well.

“It may also be that people are more likely to speak to Zi Teng now and to report these incidents.”

Kapai said the government may have wanted to be seen cracking down on illegal activities related to prostitution because of the Legco elections earlier this year.

“I think there have definitely been attempts to find areas where things are being improved. But I have to have reservations about whether the strategy has been effective. I am not sure the political will is ­really there.”

And commenting on the reported tendency by police to target suspected migrant sex workers, she said this may have emerged against a backdrop of xenophobic messages being circulated around the world.

“It likely stems from the rhetoric we have seen around the notion of ‘illegal immigrants’ globally”, she said.

“There is a change in the tone that supports such kinds of police tactics.”

A police spokesman said sex workers who felt they had been victims of unfair treatment by officers should make a formal complaint.

“The primary objectives of police enforcement actions are to prevent exploitation of those engaged in prostitution, combat organised prostitution activities, and minimise the nuisance caused to members of the public,” he said.

“To exercise stringent control over the conduct of police officers in mounting operations combating vice activities, the internal guidelines require officers at the rank of senior superintendent to closely supervise every operation that involves police agents, to ensure that the tactics employed in gathering ­evidence, including the extent of body contact with sex workers, are strictly necessary and proportional to the purpose of the operation.”

The spokesman added: “Any persons dissatisfied about police enforcement operations may lodge a complaint, and the Complaints Against Police Office will handle all reports or complaints in confidence and in a thorough, impartial, professional and ­proper manner in accordance with the Independent Police Complaints Council Ordinance.”