Several hundred suspected paedophiles including doctors, teachers and former police officers have been arrested in the biggest ever UK crackdown on obscene images of children.
The unprecedented six-month operation headed by the National Crime Agency (NCA) saw 660 people held for downloading and sharing the sickening pictures, and has already led to charges for serious sexual assault.
Those arrested included a doctor who had access to more than one million depraved pictures, was found to have met up with boys and kept sex aids and rope in the boot of his car.
Scout leaders and care workers were also among the huge number of people held across England, Wales, Scotland and Northern Ireland, and the vast majority had never before aroused suspicion.
The massive investigation, involving all 45 British police forces, led to 431 children who were were in the "care, custody or control" of the suspects being "safeguarded", including 127 who were identified as being at serious risk of harm.
Those arrested had used the internet or the so-called "dark web" - internet content that is not listed for access by normal search engines.
NCA deputy director general Phil Gormley said: "This operation has been about protecting children who are victims of, or might become victims of, sexual exploitation.
"Children are victimised not only when they are abused and the images first taken, but at every subsequent time that image is viewed by further offenders or distributed."
Mr Gormley said he was "profoundly disappointed" that so many suspects had been arrested over this type of crime.
He said: "The alternative is not to look under the stone, and we cannot afford not to look under this stone.
"There are very significant volumes of people viewing this material in this country and abroad. We are going to need to understand as a society how we are going to confront this issue.
"We are not going to be able to arrest our way our of it. The numbers are significant, the volumes are huge."
The NCA would not reveal the precise tactics it had used, but in previous child abuse cases officers have gone undercover and posed as potential victims to lull sex offenders into showing their true colours.
Chief Constable Simon Bailey, who is the national lead for child abuse investigations, said that police can track paedophiles online, even when using the dark web.
"Law enforcement now has the capability to see what people are doing," he said. "Six hundred and sixty people have currently been arrested, there will be more arrests. There is a clear message to anybody using the internet to facilitate and to commit this type of crime that you are vulnerable."
There were only 39 registered sex offenders among those arrested, with the majority able to avoid detection until now.
One of the suspects said he had been viewing images of child abuse for 30 years and had repeatedly travelled to south east Asia as a sex tourist.
When asked how hundreds of sex offenders had escaped detection until now, Mr Gormley said: "It's a bit like a drugs problem. You need to look for it if you're going to find it. People are unlikely to report this type of crime, you're not going to have witnesses to it in the way that traditional crime types will."
Two years ago the NCA estimated that 50,000 people in the UK were involved in sharing child abuse images online, and in the past 20 years the number of images available has soared from an estimated 10,000 to tens of millions.
Mr Gormley went on: "I am pretty appalled about what it says about human nature. Which is why we need to think very carefully about what this means and how we approach this type of offending behaviour, and the propensity of quite large numbers of people to view this material."
So far officers have searched 833 properties and examined 9,172 computers, phones and hard drives.
The NCA said it built up "intelligence packages" on suspects and sent them to police forces across Britain before arrests were made.
Claire Lilley, head of online safety at the NSPCC, said: "Direct action like this sends a strong message to those who subject children to harrowing sexual assaults that they can and will be traced and prosecuted.
"But law enforcement agencies alone cannot deal with the vast problem of illegal images which continue to flood the market. Industry has to find inventive ways of blocking the flow of such horrendous pictures which are only produced through the suffering of defenceless children - many of who are not even old enough to go to school.
"So while this operation must be rightly applauded we should view it as yet another warning sign that far more needs to be done if we are to stem the sordid trade in these images, which are often used by those who go on to abuse children."
Speaking during a visit to a business in west London, Prime Minister David Cameron said: "I think what these figures, what these arrests, show is that the National Crime Agency, which this Government set up, is now up and running and doing a very good job at bringing all the modern methods of policing and technology together to go after the perpetrators of a particularly horrible crime.
"Now it's important that these cases are bought to trial and tried fairly, but it's a good step forward for the National Crime Agency and I commend them for their work."
The suspects arrested included six teachers, four members of school staff, two members of police staff, two medics and one person employed by social services.
There were also two ex-police officers and one person who worked with children in the private sector, as well as a Scout leader.
They are all aged from their 30s to 50s, the NCA said.