Children as young as 11 have been caught carrying knives in Wales, research by BBC Wales has found.
Officers in south Wales, north Wales and the Gwent force all caught children that age with a weapon.
South Wales Police also seized a pistol and a handgun from children at school in 2015 and 2017 respectively.
Its Chief Constable Matt Jukes warned children were being drawn into a "culture of carrying knives".
Dyfed-Powys Police was also called to reports of an eight-year-old with a knife in 2014, but no weapon was found by officers and no offence was proved.
Insp Mike Melly said: "There is no evidence to show that children carrying knives is a problem for Dyfed-Powys Police and we are not seeing an increase in reports of this kind."
Mr Jukes said youngsters told police they carried a knife for self-protection, while some thought it was "street or cool".
He said officers were carrying out talks in schools and on the streets about the consequences of knife crime, but said some children felt they needed to do it for self protection and they "need to do it".
Det Insp Abi Biddle, who is based in Cardiff, said she had seen an increase in 11 to 19-year-olds carrying knives, "not necessarily in schools but in communities".
"The 15 to 24 bracket is higher, but there are pockets where you get the odd 11 or 12-year-old carrying a knife," she said.
"They think it's fine. They see it as 'my mates have got one, so I've got one'."
Mr Jukes said there had also been a rise in young drug dealers coming on trains from London, sometimes armed with knives.
"Now that's a relatively new phenomenon for us and what it's bringing with it is some of the background of further violence in those groups that we wouldn't have historically seen around our local drugs gangs," he said.
He said this was against a background of an overall increase in knife crime, but noted that a third of incidents involving a knife were related to domestic abuse.
Mr Jukes added: "It's important to say compared with other parts of the UK that rise is not as significant, but it's really significant for us here."