Police are only "skimming the surface" despite efforts to protect children online, the Metropolitan Police Commissioner has said.
Sir Bernard Hogan-Howe said that while the force had employed more officers in the area, policing the internet remained a "massive challenge".
The commissioner said the Met had a squad of 300 officers dedicated to internet security which is likely to grow by 200 in the next year, but added "even then I think we're skimming the surface a bit".
He said the "real logistical problem" was juggling online crime with other police roles.
"We're trying to do all these other things as well: the pubs all turn out tonight and there will be fights, there will be big armed robberies, there will be anti-social behaviour, there will be 70,000 marchers in London - everybody expects all of this doing... there's a risk assessment process that says in these cases what's the risk of that person hurting a young person today."
Sir Bernard - who was grilled by a panel of young people on Sky News' Hotseat programme and the social affairs editor Afua Hirsch - said funding cuts had hit the Met's ability to tackle the array of crimes.
"One of the problems we do have is that you get less and less money spent on policing at the moment. Probably a third less over the next two years, so there's going to have to be some hard decisions made by society and by the police.
"It's incredibly important but there are also other things too which we have to see to so we do our best to put as many resources as possible to work our way through it but there's no doubt its a massive challenge."
He added: "There's no doubt its a challenge to keep up with the pace, because before the internet existed this sort of crime didn't happen. "
Sir Bernard also said that police arrest an average of one person a day for terrorist offences and that the number arrested had risen by a third compared to last year.
Britain's most senior police officer told the group of young people: "The number of operations that we have been carrying out has increased by about third. So on average we've arrested one person a day throughout the United Kingdom for terrorist offences.
"And we've disrupted some significant plots, some awaiting court now, where they would have hurt people had they carried on."