SEOUL – A South Korean man was sentenced to 34 years in prison Thursday before being forced into pornography as part of the country’s crackdown on an infamous network of online chat rooms that lured young women, including minors, with promises of high-paying jobs.

The man, Moon Hyeong-wook, opened one of the first such websites in 2015, prosecutors said. The 25-year-old Mr. Moon operated a secret, members-only chat room under the nickname “GodGod” in the Telegram Messenger app and offered more than 3,700 clips of illegal pornography.

Mr. Moon, an architecture major who was expelled from college after his arrest last year, was one of the most notorious of the hundreds of people arrested by police in the course of their investigation. Another chat room operator, a man named Cho Joo-bin, was sentenced to 40 years in prison last November.

“The defendant did irreparable harm to his victims through his crime against society that undermined human dignity,” said presiding judge Cho Soon-pyo in his ruling on Mr. Moon on Thursday. The trial took place in a district court in Andong City in central South Korea.

Mr Moon was charged in June of forcing 21 young women, including minors, to make sexually explicit videos between 2017 and early last year.

He targeted young women looking for high-paying jobs through social media platforms and then lured them into making sexually explicit videos, which prosecutors said promised high payouts. He also hacked into the online accounts of women who had uploaded sexually explicit content and pretended to be a cop investigating pornography.

Once he received the pictures and personal information, he blackmailed them to blackmail the women and threatened to send the clips to their parents unless the victims provided more footage, prosecutors said.

The prosecution asked for a life sentence for Mr. Moon.

Last December, police announced they had investigated 3,500 suspects, most of them men aged 20 or over, as part of their investigation into online chat rooms used as avenues for sexual exploitation and pornographic dissemination. They arrested 245 of them.

The police also identified 1,100 victims.

The scandal, known in South Korea as the “Nth Room Case”, sparked outrage over the cruel exploitation of young women. Women’s rights groups struck courthouses where chat room owners were on trial and accused judges of condoning sex crimes by imposing what they considered light sentences.

On Thursday, lawyers held a rally outside the Andong Courthouse demanding the maximum sentence for Mr. Moon.

In recent years, South Korean police have taken action against sexually explicit file-sharing websites as part of an international effort to combat child pornography. As smartphones proliferated, they quickly found that much of the illegal trade migrated to online chat rooms through messaging services like Telegram.

Police said they were having trouble tracking customers in the online chat rooms because they often used cryptocurrency payments to avoid getting caught.