The officer repeatedly strikes the 17-year-old on the legs as he says: 'I'm a child!'
Members of the public watching the scene in Romford, east London, can be heard criticising the officer who had arrested the youngster for possession of Class B drugs with intent to supply.
The Metropolitan Police said there was a 'struggle' before his arrest and a second teenager assaulted an officer.
The video, which emerged online on Tuesday, has prompted former Deputy Mayor of London, Lee Jasper, to announce he will make a formal complaint to the Metropolitan Police.
The incident happened on Monday in Heath Park Road.
The video, which does not show the whole incident, shows a young man wearing a set of handcuffs being escorted by a police officer in plain clothes with a baton primed for strike.
The officer then strikes the teenager's legs and an onlooker can be heard shouting 'what the f***, fam'.
The officer continues to batter the teenager who can be heard shouting 'what the f*** are you doing to me, why the f*** are you hitting me for?'
Ther handcuffed teenager then gets on his knees and pleads: 'I'm a child, I'm a f****** child.'
The police officer then holds the detained teenager down and turns to the onlookers and says: 'Someone dial 999.'
An onlooker says: 'You can't just hit him like that,' while the detained teenager screams: 'I've done nothing, he's slapping me with a stick.'
The youngster then turns his attention to the officer and says: 'Do you know what you have just done? I'm taking you to court.'
A Metropolitan Police Service spokeswoman said: 'Police were on patrol in Heath Park Road, Romford on Monday, 22 April when they found reason to stop and search two teenagers under the Misuse of Drugs Act.
'The second male, aged 14, escaped the officers and ran to his home address. Here, an officer found the male and after another struggle, he was detained. During the struggle the officer was assaulted by the male.
'He was arrested for obstruction and assault on a police officer. Both males were taken to an east London police station and subsequently released under investigation.
'We are aware of footage circulating on social media. No formal complaint has been received at this time but the full circumstances surrounding the incident, including the actions of the officers, are being reviewed.'