The mother of a black 13-year-old boy says she feels "betrayed" after he was surrounded by armed police when his water pistol was mistaken for a gun.
The boy was handcuffed and confronted by marksmen in Hackney, east London, in July, the Alliance for Police Accountability (APA) said.
His mother says she is "extremely angry" and the incident has been "a deeply traumatic experience".
The Met Police's Det Ch Supt James Conway apologised for "trauma" caused.
The boy was having a water fight with his sibling on 19 July when a police officer on patrol reported a potential firearms incident, the APA said.
A police van then "rammed" Child X off his bicycle, the APA said, knocking him to the ground before he was "surrounded by armed police officers who pointed their firearms at him" and arrested him on suspicion of being in possession of a firearm.
He was "de-arrested" at the scene soon afterwards, it said.
One of the water pistols being played with was blue and white and one was pink and white, and unmistakably toys, it added.
'Described me as aggressive'
The child's mother said: "The attitude of the police to him - and to me - is shown up in the words of the senior officer at the scene when I protested to him about the conduct of his officers: he told me I was lucky that they had not arrested my son.
"For what? For playing with a brightly coloured plastic water gun with his younger sibling on the streets behind our home? For being a black boy on the streets of Hackney?
"I know - and the police know - that they would not have treated my son in the way they did if he had been a white 13-year-old boy.
"I know that they would not have treated me with the contempt shown towards me or described me as 'aggressive' if I was not black."
She added she was "in shock" and feels "broken by it all; distraught because I was not able to protect my child from what happened".
Speaking about her children, she continued: "How do I help them cope with the fact that the police present at the scene seemed unable or unwilling to apologise, explain or answer for what they had done?
"And after what has happened, how can I ever tell them that they can turn to the police for help?"
The APA added the "treatment of Child X and the horrific scene that unfolded was appalling" and the boy's mother had been "treated with contempt by the officers".
'No officer misconduct'
The boy's mother added that after contacting her MP she was met by the borough's police commander for her to view the officers' body-worn camera footage but it was "too painful and traumatic to see what they did to my son".
She twice asked the police watchdog, the Independent Office for Police Conduct (IOPC), to investigate what happened but said it refused, "insisting that the investigation must be carried out by the police themselves".
An internal investigation by the police force found no misconduct had been committed by the officers involved.
A further complaint accusing them of racial bias is still being investigated by the Met's Directorate for Professional Standards.
The boy's mother concluded: "I feel let down and betrayed, not only by the police, but also by the IOPC, and by the whole system that is supposed to look after our children, black or white: they deserve better."
Det Ch Supt Conway said he had apologised to the family for the "extremely distressing" incident.
"We know it may cause public concern and we want to help the public understand why we responded in the way we did," he said.
He added that, according to the College of Policing, officers "should treat all firearms as real and loaded until proven otherwise".
"This does not in any way detract from our recognition of the trauma caused to the boy, for which I apologised soon afterwards to his family," he added.
A City of London Police spokesperson said the force attended the incident in support of the Met Police "as common practice".
