Up to 27 women and girls, including at least four still at school, were forced to have sex with a man who posed online as a woman doctor, police said.
The construction worker persuaded the girls to send him nude photographs in return for free consultations, then threatened to release the pictures if they did not have sex with him, a senior inspector said. The four schoolgirls, aged 13 to 17, were among dozens said to have fallen for the scam.
A 28-year-old was arrested in connection with the case at his home in Tsing Yi yesterday.
Wong Chi-kin of Sham Shui Po anti-triad squad said police had been able to contact 17 of the 40 victims and appealed for the others to come forward. Not all of them had sex with the man.
Wong said the suspect got to know the victims through mobile messaging services, telling them he was a female doctor with knowledge of gynaecology.
"He told the victims he could diagnose health problems by looking at naked photographs. They believed him and sent him their nude pictures," he said.
After receiving the photos, the man asked the victims to visit him for treatment and told them that their health problems could be treated by sexual intercourse.
"Those who refused were threatened that their nude photographs would be made public unless they had sex with him," the senior inspector said.
Among the 17 victims who police had contacted, 15 were aged 13 to 21. Four of them said they had been taken to love hotels to have sex with the man.
The case came to light when a 16-year-old girl sought help from Sham Shui Po police on October 28, saying the man was demanding sex and threatening to publish her nude photographs.
He was picked up by police when he met her in Sham Shui Po the same day and granted bail because police needed more time to investigate.
During the investigation, police found 40 albums on the man's mobile phone containing nude photographs of the victims. He was rearrested yesterday.
Wong said the suspect may face charges of rape or procuring sexual intercourse by threats and false pretence.
Last night the man was being detained at Cheung Sha Wan police station for questioning and no charge had been laid.
In the first eight months of the year, police handled 83 reports of rape, an increase of nearly 19 per cent from the 70 during the same period of last year.