"I sent him the video with the bleeding, and told him that it hurt so much and I couldn't do it anymore. I got a reply about 10 minutes later. Just do it anyway. So, I kept doing it and filming it and sending it ... Even still, it hurts. My heart hurts a lot. It was just too much pain."

The words of a South Korean middle school student, underage at the time she was blackmailed, describing to a local media outlet how she provided about 40 explicit videos to a chat room sex ring known as the Nth Room because she was told her personal details and images would be released if she failed to comply.

South Korean Police have arrested 24-year-old Cho Ju-bin, who they claim is the central figure in the chat room ring that mostly used Telegram, an encrypted social media site.

With his hands tied and arms bound with rop,e Cho was made to face the public after five million people signed a petition demanding the police identify the key suspect.

"I apologies to everyone who has suffered because of me. Thank you for ending the life of a demon that I couldn't stop," said Cho, standing at the top of the steps at a Seoul police station as demonstrators below held banners calling for a tough sentence.

It's alleged the Nth Room ring blackmailed more than 70 women, including 16 underage girls.

Cho and others allegedly tricked them with fake job adds into sending revealing photos, before threatening to release the images if their victims did not send even more explicit material, including violent acts such as cutting the word "slave" into their skin.

The case has galvanized South Koreans, more than a million and a half of whom have called for all 260,000 users of Cho's chatrooms to be publicly identified.

Cho, and others, are alleged to have charged members of their chat rooms the equivalent of up to 1,200 U.S. dollars per shared image. 

Many South Koreans are calling for tougher laws that better protect them from cybercrime and provide stronger penalties for perpetrators as well as for people who pay for, or even share, illegally obtained images.

"A first sentence is very light, three years. I don't think it makes any sense at all.  There's a lot more cases like the recent celebrity sex case. They're not punished by the law at all," says Baek Jung-soo, a Seoul resident, who, like many South Koreans, has been angered by the case.

"I'm a mother and a daughter, so I can understand the anger of the young towards men right now," says Baek's friend, Lee Mi-soon.

"The people handing down the punishments are mostly men. So, I think there is a bias in the punishment," says Lee, adding "I hope there will be stronger penalties to prevent such a crime from happening again."

The case has drawn so much attention here even the office of President Moon Jae-in issued a statement after the president described the crimes as "a cruel act that destroyed human life".

"President Moon asked the police to treat the case as a serious crime and conduct a thorough investigation to strictly punish the offenders, and he said that digital sex crimes involving children and teenagers especially, should be dealt with more sternly," Kang Min-seok, South Korea's Presidential Blue House spokesman told reporters as the Blue House.