South Korean police arrested three teenage males on suspicion of sexually molesting primary school students in a series of assault cases that a civic group and local media said had more than 50 victims.

Older students forced boy and girl students at the primary school in the southeastern city of Daegu to mimic sexual acts they had seen on pornographic websites and TV, according to the civic group made up of parents from the area, teachers and human rights advocates.

News of the case has saturated South Korean media in the past few days and led to questions about the responsibility of parents and educators.

A police official in Daegu said the three middle school boys were arrested on suspicion of sexually abusing eight elementary or primary school girls. The official would not comment on the scope of the investigation.

The three are part of a group of 11 students who sexually abused the children on the campus of the elementary school where students are mainly from seven to 12 years old. The students would beat the younger students if they did not participate, Yonhap news agency reported parents and police as saying.

The school was first told of the sexual violence months ago, local media citing police sources said, and has been criticised for not acting swiftly.

The name of the school has not been released in order to protect the children.