Madang Police Commander Anthony Wagambie confirmed a report in The National newspaper that said the cult members allegedly ate their victims' brains raw and made soup from their penises.
"They don't think they've done anything wrong; they admit what they've done openly," Wagambie said.
He said the killers believed that their victims practiced "sanguma," or sorcery, and that they had been extorting money as well as demanding sex from poor villagers for their supernatural services.
By eating witch doctors' organs, the cult members believed they would attain supernatural powers and literally become bullet-proof, he said.
"It's prevalent cult activity," Wagambie said. He said he believes there could be between 700 and 1,000 cult members in several villages in Papua New Guinea's remote northeast interior. All of them might have eaten human flesh, he said.
According to the report in The National, which is published in Papua New Guinea, 28 men and women appeared in a Madang court on Tuesday. Wagambie said they were charged with willful murder.
It was not clear what happened to the 29th suspect. Murder is punishable by death in Papua New Guinea, a poor South Pacific island nation.
Wagambie said the suspects were not required to plea to the murder charges and were being held in custody.
Police will gather more witness statements before pressing charges related to the cannibalism allegations, he said.
Cannibalism was part of traditional culture in Papua New Guinea, where human flesh was known as "long pig," and survived in isolated pockets into the latter part of the 20th century while the country was under Australian colonial rule.
Wagambie, 36, said he had never heard of a previous case of cannibalism in his lifetime.
He expected police would make around 100 arrests over the weekend for cult-related crimes.
Four of the seven victims were murdered last week, Wagambie said, adding that no remains had been recovered.
"They're probably all eaten up," he said.
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