A doctor working in paediatrics has admitted a string of sex offences against children.
Raza Laskar, 32, of Ashton-under-Lyne, Greater Manchester, pleaded guilty to a number of charges including sexual activity with a child and causing or inciting child pornography.
Greater Manchester Police said the charges relate to offences against 12 boys aged under 16-years-old.
Some of his victims were based in the UK while others lived in foreign countries.
Detectives were alerted to Laskar's activity when over 1,600 files suspected to contain indecent photographs of children were made available on a file sharing site between Christmas Day last year and January 27.
Investigators linked the IP address to Laskar's home and a search was carried out on May 2.
Officers seized a number of computers and data storage devices which were examined by GMP's high tech crime unit. An external firm was also commissioned to carry out the work because of the large amount of data recovered.
This included over 1.2 million images and videos of Laskar engaging in sexual acts with a child in a hotel room.
Hundreds of chat logs and videos were reviewed which led to officers identifying more children groomed and incited by Laskar to engage in sexual activity.
Detective Inspector Theresa Carter of Greater Manchester Police said the most sinister aspect of the case was that Laskar used his knowledge as a children's doctor to 'ingratiate himself' with the boys.
"Obviously he has experience in dealing with children and knows how children tick," she said. "The children knew he was a doctor because he told them and he knew the boys were all under 16.
"He would advertise on sites using details like '30-year-old male wanting slaves' on sites like Topix. Some people that contacted him were just lonely young boys who wanted to talk. He was quite cruel in the way he did things as he knew what he was doing. The sinister side is the fact he is a doctor and he understands children."
Laskar admitted six counts of causing or inciting child pornography, one count of taking indecent photographs, eight counts of causing or inciting a child to engage in sexual activity, five counts of voyeurism, three counts of sexual activity with a child, two counts of distributing an indecent photograph of a child, one count of incitment to distribute an indecent photograph of a child, two counts of causing or inciting a child under 13 to engage in sexual activity, two counts of making indecent photographs of a child and one count of possessing an extreme pornographic image.
The charges relate to offences against 12 boys under 16-years-old.
He will be sentenced on 9 January 2015.
Police confirmed he was caught as part of Operation Notarise, which started this year.
In April 2014, GMP received information from the National Crime Agency that between Christmas Day 2013 and 27 January 2014, 1,605 separate files, suspected of containing indecent photographs of children, had been made available on a file sharing site.
The IP address was checked and an address in Ashton-under-Lyne identified and on 2 May 2014, a warrant was executed at the house.
A number of computers and data storage devices were seized and subsequently forensically examined by GMP's high tech crime unit and latterly by an external company; such was the amount of data recovered.
Chat logs were recovered as well as videos of Laskar engaging in sexual acts with a child that had been taken in a hotel room he had booked.
Further examination of the vast amounts of data recovered revealed more than 1.2million images.
In addition hundreds of chat logs were reviewed and videos recovered that led to officers identifying other child victims whom Laskar had groomed and incited to engage in sexual activity.
The children were found after examination of internet chat logs and the investigation team traced them using their usernames, emails and IP addresses.
Some victims were based in the UK and others elsewhere around the world.
DI Carter said: "Raja Laskar is a person who lived two very different lives.
"Like Jekyll and Hyde, he is vastly different in moral character depending on the situation, going from caring for and treating children to deliberately targeting, grooming and abusing them.
"He has admitted a vast number of appalling child sexual abuse offences and the size and the scale of this operation obviously came as a shock to the investigation team when you consider his background allied with the scale of his offending.
"His profession will obviously be an aggravating factor and one that many people will be extremely concerned about.
"While I can stress that we found no evidence that he committed any offence during the course of his employment, it will be of absolutely no comfort to the boys and the families of the boys he targeted online.
"The one thing I would like to highlight on the back of this is internet security and awareness.
"It is so important, in this digital age where even young school children have the internet in the palm of their hand that they know about the dark side of the web.
"There are dangers lurking if the right precautions are not taken and I would encourage parents to talk to their children and discuss what websites they are visiting, who they are talking to and what to do if approached by someone who asks them to engage in sexual activity."
A NHS England spokesman said: 'The charges against Dr Laskar did not relate to his work. However, we would like to extend our sympathies to the victims and families involved in this case. "
They are urging anyone with any concerns to contact them.
Police forces across the country have arrested 660 suspected paedophiles including doctors, teachers and former police officers in the biggest operation for more than ten years targeting online child abusers.
Only 39 of those arrested during the six-month operation were registered sex offenders and the majority of them were not known by the police. The allegations range from possessing indecent images of children to serious sexual assault.
The operation targeted people accessing indecent images online with the focus on a number of people who had unsupervised access to children in the course of their work. They were identified through examinations of the so-called dark web – the encrypted underbelly of the web which gives users greater anonymity – and through traditional online methods of sharing files.