The leader of a South Korean doomsday cult who held 400 people captive in Fiji and subjected them to violent beatings has been sentenced to six years in jail.

Shin Ok-ju, founder of the Grace Road Church, convinced her followers to move to Fiji in 2014, which she said was the “promised land”, pointed to in the Bible, where they would survive coming apocalyptic events.

Once they arrived on the island their passports were confiscated. Those who left the group reported brutal rituals, called “threshing floors” in which people were beaten as punishment for sinful actions or to drive out evil spirits.

On Monday a South Korean court found Shin guilty on multiple criminal charges including violence, child abuse and fraud.

“The victims suffered helplessly from collective beatings and experienced not only physical torture but also severe fear and considerable mental shock,” said the Anyang sub-court of the Suwon district court.

“Heavy punishment is inevitable against illegal acts carried out in the name of religion,” it added in a statement earlier this week.

Shocking footage showing Shin beating her followers and ordering them to beat one another was shared with the Guardian by South Korean police last year.

In several videos, Shin was shown calling members of the church forward during her sermons and then hitting them in the face, pulling and cutting their hair and throwing them to the ground.

She was arrested along with three other church leaders when they landed at Incheon airport just outside the South Korean capital Seoul in July 2017.

People have joined Grace Road Church and travelled to join the group in Fiji from all over the world.

In an interview with the Guardian, one American teenager who was taken to Grace Road Church by her mother, told how she was trapped there, had her passport taken, her medication withheld and all contact with her father and sister in the US cut off. She eventually escaped by running out of the church and making a phone call from a convenience store.

Grace Road Church still owns and operates multiple businesses across Fiji, including farms, cafes and construction companies.

After the sentence was handed down, an opposition leader in Fiji called for an investigation into alleged links between the Fijian government and the cult, according to RNZ, saying that as a member of the UN Human Rights Council, the government should do more to investigate concerns about modern-day slavery in Fiji.