More than 600 suspected paedophiles, including doctors and teachers, have been arrested across Britain after a six-month investigation led by the National Crime Agency.

The agency, dubbed Britain's FBI, announced on Wednesday it had arrested 660 suspected child abusers and safeguarded more than 400 children in the first nationwide investigation of its kind.

Of the 660 arrested, 39 were registered sex offenders but the vast majority of suspected paedophiles were off the police's radar. Those arrested include doctors, teachers, scout leaders, care workers and former police officers.

The operation, which had remained secret until Wednesday, targeted people accessing indecent images of children online.

Phil Gormley, the NCA deputy director general, 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."

One of those arrested and subsequently charged was a doctor who allegedly kept more than 1m images of child abuse on his computers and had arranged to meet boys.

In another case, a foster carer with no previous convictions or allegations of offending was caring for a vulnerable child when he was arrested. Four computers and one phone have been seized by police and the suspect has attempted serious self-harm, detectives said.

Gormley said the police operation was about influencing potential offenders before they "cross the line" from accessing indecent images to directly sexually abusing children.

"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," he said.

"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."

Chief constable Simon Bailey, the national policing lead for child protection and abuse investigations, said the vast majority of forces in England and Wales were dealing with an unprecedented increase in the number of reports of child sexual abuse.

He said: "Sexual abuse is a complex crime taking many forms. Forces are investigating exploitation of children and young people by groups and gangs, non-recent abuse including large-scale investigations into abuse in institutions over many years and sexusal abuse by parents and family members.

"During this operation we've targeted offenders accessing child abuse images. Police must continue to use a range of investigative techniques targeting all forms of abuse if we are going to protect children and bring offenders to justice. Chief officers are committed using all the tools available to them because nothing is more important in policing than protecting vulnerable people."

Another suspect admitted when interviewed by police that he had viewed child abuse images for three decades, since he was 16. The man also admitted that he travelled to Vietnam, Cambodia and Thailand "for sexual purposes".

Detectives said that one suspect was part of an online group that shared films in which he abused a child, including footage featuring "the most serious form of abuse against very young children".

Another suspect, whom police described as a registered and violent sexual offender, was already being prosecuted over the rape of a 13-year-old girl when police officers discovered a stash of child abuse images at his home. The images displayed a wide range of abuse, police said, ranging from "erotic posing through to the most severe forms of abuse".

Greg McGill, the head of organised crime for the Crown Prosecution Service (CPS), said: "These abhorrent offences only contribute to the cycle of harm suffered by the victims in such cases. Specialist CPS prosecutors continue to work with the NCA, police partners and other relevant agencies in the fight against child sexual abuse and exploitation.

"We are committed to helping safeguard young people and, wherever we are able to, we will prosecute where there is sufficient evidence and it is in the public interest."

Donald Findlater, the director of research and development at the Lucy Faithfull Foundation and Stop it Now!, said: "The Stop it Now sexual abuse prevention line is taking an increasing number of calls from men who are concerned about their online behaviour as well as from their families and friends. Thousands of men have called following their arrest for possession of indecent images of children. Thousands of (mostly) women have called because of a partner's arrest or simply because of a concern about their partner's sexual behaviour towards children."

The Stop it Now helpline provides confidential advice, support and help to try to keep children safe. It supports those who have offended online to take responsibility for abusive behaviour and advises their family and friends on how to manage future risk.

Findlater said: "However many are arrested for possession of indecent images, there are hundreds of thousands more who have viewed such images and have not yet been arrested.

"We urge all such men – young and old alike – to contact the helpline for help to stop their illegal and harmful behaviour.

"Not only do children continue to be harmed by this inappropriate use of their images, but also the offender risks losing friends, family, job, reputation and more should their offending come to light. Call 0808 1000 900 for confidential help before it is too late."

• This article was amended on 5 August 2014 because an earlier version said the National Crime Agency had taken more than 400 children into care. This has been corrected to say the agency has safeguarded more than 400 children.

