An operation by the National Crime Agency involving 45 police forces has seen 660 suspected paedophiles arrested across the UK.
Thirty-nine of the 660 were registered sex offenders but the vast majority had not come to the authorities' attention before. More than 800 properties were searched.
One arrested man had access to 17 grandchildren - two of them had already made allegations against him.
Two men, one of whom is a doctor, had between them more than a million indecent images of children on their phones and computers.
Care workers and former police officers were also among the hundreds arrested.
As a result of the six-month investigation across England, Wales, Scotland and Northern Ireland - which has only now been revealed - more than 400 children have been safeguarded, the NCA said.
A total of 431 children now under official protection had been in the "care, custody or control" of suspects, 127 of those children were considered to be at immediate risk of harm, the agency revealed.
None of those arrested is a serving or former MP or member of the Government, it said.
Charging decisions are still due in most cases because of continuing inquiries, but charges brought so far range from possessing indecent images of children to serious sexual assault.
The NCA's deputy director general, Phil Gormley, said: "This is the first time the UK has had the capability to coordinate a single targeted operation of this nature.
"Over the past six months we have seen unprecedented levels of cooperation to deliver this result.
"Our aim was to protect children who were victims of, or might be at risk of, sexual exploitation. A child is victimised not only when they are abused and an image is taken. They are re-victimised every time that image is viewed by someone.
"Some of the people who start by accessing indecent images online go on to abuse children directly. So the operation is not only about catching people who have already offended - it is about influencing potential offenders before they cross that line.
"We want those offenders to know that the internet is not a safe anonymous space for accessing indecent images, that they leave a digital footprint, and that law enforcement will find it."
Mr Gormley said he was "profoundly disappointed" at the scale of the problem and the number of arrests police were forced to make.
It was necessary, he said, to take a harder look at the amount of people looking at child abuse images on the internet.
"The alternative," he said, "is not to look under the stone, and we cannot afford not to look under this stone."
Claire Lilley, Head of Online Safety at the NSPCC said: "This is an important two-pronged operation which has rescued children from abuse and also identified many previously unknown sex offenders.
"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."