reported.
It was reportedly sparked by a 16-year-old Wapping High School pupil allegedly calling Katelynn a "slag" before trying to kick her.
Katelynn's horrified family have blasted the police for using unnecessary force - and the Independent Police Complaints Commission (IPCC) has been called in to investigate.
Joan Whittaker, 51, Katelynn's grandmother, said: "This policeman wasn't tackling a grown man, this was a little girl of 13.
"It was extreme force and it absolutely was not justified."
She added: "My grand-daughter lifted her hand as if to slap this girl.
"But there was no physical contact with these two kids, it was just two kids having a dispute and the policeman went too far.
"He came up behind my grand-daughter.
"Katelynn didn't know he was there and he restrained her on the ground and on her side in handcuffs.
"She's not struggling, she's compliant. She tries to get up and he just drags her like a piece of meat."
Katelynn was taken home following the incident, on October 15, and was not arrested or questioned.
The youngster was left with badly bruised and swollen knees, a lump on her head, and severe bruises to her arms and torso - while her twin sister alleges the officer also tried to threaten her into deleting the damning video clip.
An IPCC spokesman said: "The IPCC received a referral from the Metropolitan Police concerning an incident outside Wapping High School on October 5.
"Following a careful assessment we have decided that the incident should be locally investigated by the Metropolitan Police."
A statement from Wapping High School said: "The police were called to an incident outside of school concerning the conduct of a student from another school in order to safeguard our students and members of the public.
"The matter is currently in the hands of the police."
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