A 13-year-old boy was surrounded by armed police officers after one mistook his water pistol for a real gun.
A police van knocked the teenager off his bike and he was handcuffed as officers pointed their weapons at him in Hackney, east London in July.
He was arrested on suspicion of being in possession of a firearm but was subsequently de-arrested.
The Alliance for Police Accountability (APA) condemned the treatment of the boy, known as Child X, saying the water pistol was unmistakably a toy.
He was having a water fight with his younger sibling on 19 July when an on patrol officer reported a potential firearms incident.
One of the water guns was blue and white, and one was pink and white, the APA said.
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Armed units from the Met and City of London Police were sent to the scene, according to the APA.
It said in a statement: “The subsequent treatment of Child X and the horrific scene that unfolded was appalling.
“A police van rammed Child X off his bicycle, knocking him to the ground.
“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, once his mother had arrived and challenged what was happening, but she too was treated with contempt by the officers when she sought an explanation for their conduct from them.”
Child X’s family have seen body-worn video of what happened and met with Detective Chief Superintendent James Conway, in charge of policing for Hackney and Tower Hamlets
Mr Conway said he had apologised to the family.
He said: “This incident was understandably extremely distressing for the boy involved as well as the rest of his family.
“We know it may cause public concern and we want to help the public understand why we responded in the way we did.
“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.”
An internal investigation by the Metropolitan Police found no misconduct had been committed by the officers involved.
A further complaint accusing them of racial bias is still being investigated by the force’s standards department.
The Independent Office for Police Conduct watchdog stated that the Met could carry out both investigations itself.
Mr Conway said: “Our officers are dealing with fast-moving situations, based on the limited information provided to them at the time.
“Such is the nature of the threat from firearms that the College of Policing is clear that officers should treat all firearms as real and loaded until proven otherwise.
“The police have a positive legal obligation under human rights legislation to protect life, which shapes our approach to responding to suspected firearms.”
He added: “I am sorry for the trauma caused to the child and I am happy to meet with the family again if they feel that would help.”
Child X’s mother will read a statement on Thursday during a press conference held by APA.
