Children as young as 10 are being pressured into sharing explicit pictures and videos of themselves online, with cases reported to police "virtually every day".
A specialist team at Scotland Yard received two reports in six days, as well as one of a child under 10 who was asked for sex while gaming online.
Detective Superintendent John Macdonald, from the force's Sexual Offences, Exploitation and Child Abuse Command, says police only see a fraction of so-called "sexting" cases.
He said: "Virtually every day we're seeing examples of very young children, I'm talking about 10 or sometimes even a bit younger than that, being asked to supply imagery online.
"It is very under-reported. I would have thought that most young girls in their younger teens to mid-teens have probably had a request for some type of image to be sent and I think they wouldn't even consider reporting it because it's so 'normal'.
"The ones that develop into more coercive requests where the child is being threatened if they don't send the image, that obviously will be less in volume, but I still think we don't see most of those being reported. It's a really hidden crime."
His team looks at cases each morning that have been flagged as potentially involving child sexual exploitation.
In a six-day period they received reports including 11 and 12-year-old girls who had sent naked pictures to men online and teenagers being threatened to force them to provide explicit images.
A poll of members of the teaching union NASUWT in March found that 63% of teachers said they were aware of 14-year-olds sexting, while 45% said those involved were as young as 13. A handful of teachers said seven, eight and nine-year-olds were involved.
In September the NSPCC said the number of children counselled by Childline about sexting had risen by 15% in a year.
Legal guidance has been updated to say youngsters in a relationship who share images between themselves should not face prosecution, but police are concerned about more sinister cases involving blackmail and abuse.
Mr Macdonald stressed how easy it is for predators to target potential victims online, starting by showering them with compliments and quickly resorting to threats if they do not comply with obscene demands.
"Any child is vulnerable," he said. "Before the internet was mainstream, it was dangerous for an offender to try and target a child because they would have to have that physical face-to-face contact, whereas now all they've got to do is sit in the comfort of their own home and do it relatively safely. They've got access to any kid.
"Any child who's got a bit of low self-esteem is vulnerable. We will also see children who have learning difficulties who are perhaps more susceptible to being persuaded to do things online.
"The public need to be aware. This isn't just a thing that happens to another child. It can happen to anyone.
"Things start off nice, there's a bit of praise, the perception of the child is then that they're trustworthy because they're being pleasant. Slowly but surely it will change over a period of time and the child is then stuck in something that they didn't think they would be."
A recent case saw Mexican national Samuel David Lopez-Florez, 25, from east London, jailed for nine years for pressuring a 12-year-old girl to send him explicit videos of herself and, when she wanted to stop, sending the footage to her friends online.
Guidance for health professionals has recently been updated to stress that concerns about a child's sexual behaviour, including sharing images, should always be followed up.
The NSPCC has also urged parents to talk to children about the perils of sexting and has issued advice about the subject on its website.