A Middlesbrough man who pretended to be pop singer Justin Bieber to trick children into sending him sex act videos has been jailed for 14 years.
Robert Hunter, 35, was caught after a girl from Tasmania informed police.
Officers found 800 videos from all over the world - some from boys as young as nine - at his home at Brough Court.
Hunter pleaded guilty to 15 charges of inciting a child to engage in sexual activity and 14 of making indecent photos at Teesside Crown Court.
The court heard Hunter spent years targeting hundreds of girls and boys across the UK, France, Spain, Italy, Serbia, Asia and Canada.
Girls, believing he was the pop star, stripped off in front of their computer cameras after he said he would be their boyfriend, the court heard.
Schoolboys also became his victims when he posed as a teenage girl and persuaded them to perform sex acts on camera which he used to lure more girls, said prosecutor Richard Bennett.
But when they tried to back out he blackmailed them with threats.
A 12-year-old girl slashed her arms after he posted indecent photos of her on Facebook along with her address and phone number.
Judge Peter Bowers said it was possibly the worst case of internet child sex abuse he had heard.
"It represents callous, almost sadistic exploitation over a number of years preying on girls' naivety who were undermined by their own behaviour," he said.
Tina Pawson, mitigating, said he was a "lonely individual who had difficulties in forming relationships with adults".
Miss Pawson said he had shown remorse and was willing to receive treatment in prison.
Mr Bennett said Cleveland Police officers spoke to forces around the world and used Interpol to trace his victims. Hunter was arrested in December 2011.
Mr Bennett said: "He was able to persuade some of them that he was the music artist Justin Bieber and he encouraged them to perform sex acts on camera and he recorded many of them."
Det Sgt Paul Higgins, of Cleveland Police, said: "Robert Hunter was a calculating and malicious predator who set out to coerce and bully young girls into exposing themselves over the internet."