Constable John Wormell, 30, allegedly attacked Ruben Jeffrey, 16, and a friend when he saw them using a motorcycle to tow a broken-down moped.
The Old Bailey court was told that the officer became suspicious when he saw Ruben slipping something into his trouser pocket, which later turned out to be a key. He grabbed him by the neck, flung him into a police van before 'raining blows' down on his face.
PC Wormell denies common assault, assault causing actual bodily harm and misconduct of a public officer.
Ruben, of Paddington, west London, was strip searched at Harrow Road police station in west London. He lodged an official complaint last August.
Maurice Aston, for the prosecution, said: 'PC Wormell saw Jeffrey slipping something into his pocket and wanted to know what it was. Jeffrey was seemingly cheeky . . .
'He grabbed him by the T-shirt and twisted it up. He said, 'Don't get fucking lippy with me because I am a police officer'.' The case continues today.