The mother of a nine-year-old boy has lodged a complaint after her son was stopped and searched by police.
Jadan Shepherd was searched while he was sitting on a wall with friends in Camberwell, south London.
His mother Sandra said she or another adult should have been present during the search. "Looking at him you can see he is not a suspect," she added.
A Metropolitan Police spokesman said the Directorate of Professional Standards was investigating the matter.
An average of five children under 10 are stopped by members of the Met every day.
Scotland Yard revealed 2,000 children under the age of criminal responsibility were quizzed last year.
Police are legally able to search children under the age of 10 for stolen or prohibited items, such as weapons and drugs.
If any such items are found they can be removed for use as evidence against others and to protect the child's safety.
Jadan, his three friends and his 13-year-old brother - the eldest boy in the group - were approached by police in Camberwell earlier this month.
"They were just sitting on a wall and a police van stopped right in front of them," Sandra Shepherd said.
"Officers got out of the van and said there had been stabbings in the area so they would need to search them."
Mrs Shepherd said all the boys were searched except Jadan's brother.
"They came home straight away and I could see something was wrong," Mrs Shepherd said.
"Jadan was angry and thinking 'what did I do wrong, why was I a suspect?'"
She added: "If they want to stop and search fair enough, but a responsible adult should have been present. I was only two minutes away."
Supt David Musker, of Lambeth Police, said it was "inappropriate" to comment on the incident while the investigation took place.
"However, I would like to reassure the communities of Lambeth that the Met takes all complaints seriously and will rigorously investigate any allegations," he said.
It is not possible for a nine-year-old to commit a criminal offence.
Dr Mike Lindsay, from the Children's Rights Alliance, said: "Let us not forget that the age of criminal responsibility, although in this country is still quite shamefully low by comparison to many other countries, is still only 10.
More than 157,290 stop and searches were carried out in London last year, nearly triple the number carried out in 2007.
Stop and searches led to 1,200 arrests in the year to September 2008.
A Home Office spokesman said stop and search was "a powerful tool" in helping to prevent and disrupt crime.
"Removing drugs, weapons or stolen goods from a child under 10 is about the safety and protection of the child as well as the detection of crime," he added.
"The police must have the powers to do both."