A 13-year-old boy was surrounded by armed police after an officer mistook his water pistol for a real gun.
The child was "rammed" off his bike by a police van and handcuffed as he was confronted by marksmen in Hackney, east London, in July, the Alliance for Police Accountability (APA) said.
It condemned the response and said the pistol was unmistakably a toy.
The Met Police's Det Ch Supt James Conway said he had apologised for the "trauma" caused to the boy.
In a statement, the APA called the treatment of the boy, referred to as Child X, "appalling".
He 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.
One of the toys was blue and white, and one was pink and white, it reported.
Armed units from the Met and City of London Police were sent to the scene, where they arrested Child X on suspicion of being in possession of a firearm, the APA said.
"The subsequent treatment of Child X and the horrific scene that unfolded was appalling," it said in a statement.
"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."
The APA said the boy's mother was "treated with contempt by the officers" when she arrived and challenged them.
Child X was de-arrested at the scene soon afterwards, the APA said.
An internal investigation by the Met 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 said the Met could carry out both investigations itself.
Mr 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.
Mr Conway said 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.
Child X's family have seen body-worn video of what happened and have met Mr Conway.
They are due to make a statement about the incident on Thursday.
A City of London Police spokesperson said the force attended the incident in support of the Met Police "as common practice".
The child was "rammed" off his bike by a police van and handcuffed as he was confronted by marksmen in Hackney, east London, in July, the Alliance for Police Accountability (APA) said.
It condemned the response and said the pistol was unmistakably a toy.
The Met Police's Det Ch Supt James Conway said he had apologised for the "trauma" caused to the boy.
In a statement, the APA called the treatment of the boy, referred to as Child X, "appalling".
He 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.
One of the toys was blue and white, and one was pink and white, it reported.
Armed units from the Met and City of London Police were sent to the scene, where they arrested Child X on suspicion of being in possession of a firearm, the APA said.
"The subsequent treatment of Child X and the horrific scene that unfolded was appalling," it said in a statement.
"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."
The APA said the boy's mother was "treated with contempt by the officers" when she arrived and challenged them.
Child X was de-arrested at the scene soon afterwards, the APA said.
An internal investigation by the Met 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 said the Met could carry out both investigations itself.
Mr 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.
Mr Conway said 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.
Child X's family have seen body-worn video of what happened and have met Mr Conway.
They are due to make a statement about the incident on Thursday.
A City of London Police spokesperson said the force attended the incident in support of the Met Police "as common practice".
