Several private Twitter accounts have been disabled after they were revealed to contain indecent images of children.
Some hacking groups are claiming to have unmasked them, the NSPCC said.
Members of the public have reported the accounts to Greater Manchester Police and North Yorkshire Police, while Ceop - the Child Exploitation and Online Protection Centre - says it is "aware".
The NSPCC asked people to "be vigilant" and report such suspicious activity.
Ceop - the policing unit dedicated to eradicating the sexual abuse of children - said it had had 25 to 30 reports of these accounts.
A Ceop spokesman said Twitter had disabled the accounts and would be reporting the find to the National Centre for Missing and Exploited Children (NCMEC) - Ceop's US equivalent - which as an American company it was legally required to.
"NCMEC will forward the account details to law enforcement in the countries where the individual concerned is," he said.
It is unclear whether the images were uploaded by a UK user, or a user based abroad.
The NSPCC said the accounts were made public in the early hours of Monday.
"To be honest, it's not a massive surprise. In our experience sex offenders will use whichever mean they can to connect with each other. They are usually quite devious," a spokesman said.
He asked people to be vigilant.
"It you see something, or are aware of something, you should report it."
As for those people storing such content, Professor Alan Woodward, of the University of Surrey's department of computing, said they were increasingly using social media rather than computers.
"If they use the web to keep any pictures then they will be able to claim it wasn't them. The weight of evidence isn't the same."
Twitter is yet to comment.