In Northern Ireland, the PSNI made 15 searches and arrested 14 people as part of the operation, which is ongoing.
Two children in the region were identified to be at potential risk of sexual exploitation and have been protected through social services.
The NCA said more than 400 children across the UK have been safeguarded as a result.
Of the 660 people arrested, 39 were Registered Sex Offenders but most of them had not previously come to the attention of authorities.
They include doctors, teachers, scout leaders, care workers and former police officers.
So far officers have searched 833 properties and examined 9,172 computers, phones and hard drives.
The targets were people accessing indecent images of children online and the majority of these people were not yet known to police – they are now, and they will stay in our sights.
PSNI Assistant Chief Constable Drew Harris
The NCA, which cannot take the lead in operations in NI, said it built up "intelligence packages" on suspects and sent them to individual police services before arrests were made.
PSNI Superintendent Rachel Shields would not elaborate on the identity of those detained, but commented: "Every profession, every gender, every culture, nobody is immune from this and it's important not to be complacent as to the type of person who could potentially be viewing these images on the internet.
"There's no doubt there will be more children out there, on a daily basis we scratch the surface, the internet has globalised offending of this nature across the world.
"It is important to understand and appreciate that every child that we come across that we can identify as living in Northern Ireland, the relevant child protect measures will be put in place."
Assistant Chief Constable Drew Harris added: "A child is victimised not only when they are abused and an image is taken but they are re-victimised every time that image is viewed by someone.
"We have identified that many offenders who start by accessing indecent images online go on to abuse children directly so the operation is not only about detecting people who have already offended - it is about reducing the risk of serious harm to children.
"Offenders possessing indecent images of children online should know that the internet is not a safe hiding place. We will continue to use a range of investigative techniques targeting all forms of abuse to protect children and vulnerable people and bring offenders to justice."
NCA 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," he explained.
"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."