Media playback is unsupported on your device Media caption A BBC News team joined one arrest operation in Norwich

More than 680 people have been arrested in the past nine months for downloading indecent images of children, the National Crime Agency has said.

More than 90% of the suspects were not known to police for similar offences, while 104 were in "positions of trust", including 32 with links to education.

Some 399 children were identified as being at risk following the arrests.

Forty of the UK's 45 police forces were involved in arrests across the country. To date 147 people have been charged.

A total of 600 premises were searched and 682 suspects have been questioned in the NCA-led operation.

Those arrested included:

23 in medical or care work

15 in law enforcement, criminal justice, armed forces or government roles

24 in voluntary positions

46 registered sex offenders

Police are using a new database of images to help trace victims.

Simon Bailey, Chief Constable of Norfolk Constabulary and the lead on Child Protection for the National Police Chiefs' Council, said officers were targeting those who they believed posed the greatest threat to children.

Analysis by Angus Crawford, BBC News

It doesn't look like much - just a computer screen in a crowded office. But according to one senior police officer, the Child Abuse Image Database (CAID) is a potential "game changer".

It contains the records of every obscene image of a child seized during investigations.

Why is that so important? The figures tell the story. In 1990 there were thought to be 7,000 such images in circulation. In one recent operation, detectives seized 2.5 million from one computer.

Any one of those images could contain a new victim unknown to police.

In the past, detectives had to painstakingly examine each one. That could take weeks and be highly traumatic. Now the process is becoming increasingly automated - known images are sifted out, leaving only ones never seen before.

The hope is that it will speed up investigations and help identify more victims, making it easier for investigators to find what one told me was that vital "needle in a haystack". A child in need of protection.

Chief Constable Bailey added: "There's a commitment from the police service to do more… more undercover officers targeting those people using chat rooms to groom children, more effort targeting people viewing indecent images of children online".

He said the NCA operation was delivering a clear message - "Don't think you can operate with anonymity in cyberspace. We will come and identify you."

'Really upset'

Police say children are increasingly being tricked into taking images themselves by predators online.

Media playback is unsupported on your device Media caption "Charlotte" was tricked into sending nude pictures of herself: "Just one click and it's gone, you don't know where"

The children's charity the NSPCC has been counselling a girl targeted online when she was 15.

"Charlotte" thought she was talking to someone her own age instead of an adult paedophile, who persuaded her to send a photograph.

"It made me feel really low and upset, at times I didn't want to be alive," she said.

"When I meet new people I always think that people have seen it or the pictures might still be out there and they may know about it."

The charity Barnardo's said it had carried out research into online grooming and suggested parents should ask their children about who they are in contact with online.