A police officer is being investigated after footage shared online appeared to show an unarmed teenager being hit with a baton.
In the clip, which shows an arrest in Romford, east London, the boy is struck on the legs several times.
The Met Police said a 17-year-old was arrested in the area on suspicion of drugs offences.
The Independent Office for Police Conduct (IOPC) said it had received a complaint.
A member of the public who witnessed the arrest contacted the watchdog to complain the use of a baton was excessive, it said.
Police said they had stopped and searched two teenage boys in Heath Park Road on Easter Monday.
A 17-year-old was found in possession of Class B drugs and arrested on suspicion of possession with intent to supply and obstruction of a drugs search.
The second boy, aged 14, made off to his home address in Brentwood Road but was later detained there, the force said.
During a struggle, an officer was assaulted and the boy was arrested on suspicion of obstruction and assault on a police officer, it added.
Both teenagers were later released under investigation.
In a statement, released through the BAME Lawyers for Justice group, the boy’s mother said she was “appalled” by the “violent arrest” and that the family had been “traumatised” by it.
IOPC interim regional director Sal Naseem said the video “only captures part of the arrest” and the watchdog would seek to establish what happened “during the entirety of the incident”.
Lee Jasper, former deputy mayor of London, called the incident a “wanton act of apparent police brutality” and said he intended to make a formal complaint.
In a statement, Scotland Yard said it was aware of footage circulating on social media and had launched a full review of the arrest.
The force said it continued “to work with our communities and will listen and act with regards to community concerns; we remain committed to keeping Londoners safe”.