Police say they believe they've helped to bring down a mainland gang that has been blackmailing men in Hong Kong by threatening to send naked photos and sex videos of them to their family members and friends.

Officers on Friday said they believe a gang based in Jiangxi province took hundreds of thousands of dollars from 31 Hong Kong victims between August 2021 and April this year.

Those targeted in the "sextortion" scam were gay men, aged between 18 and 54.

One man, 33, is said to have lost over HK$220,000, using up all his savings and also borrowing money from relatives.

Officers said the con artists tricked people they met through dating apps into downloading malicious software which stole their mobile phone contacts.

The gang then threatened to send photos and videos obtained through "naked-chats" to their victims' families and friends unless they paid ransoms in the form of gift cards.

The force's investigation stemmed from an operation to identify and remove botnet servers and phishing websites after one server was found to have been used for naked-chat blackmailing.

Superintendent Wilson Tam from the police's cyber security and technology crime bureau said naked-chat blackmail cases are on the rise and the victims are getting younger.

"People tend to know other persons through the internet or on social media platforms rather than going out to meet friends," he said.

"Using social media or the internet platforms to make friends is not a big problem. But we do need to have more publication on the protection of internet users."

Officers said mainland police have arrested 12 people in Ganzhou since April in connection with the cases.

They also believe the syndicate is linked to at least 19 similar offences in Malaysia and Singapore, and the relevant information has been passed to Interpol.

There were 708 naked-chat blackmail cases in Hong Kong in the first half of this year, 43 percent more than the same period last year. More than 60 percent of the suspected victims were aged under 30, with the youngest only 12.