A police officer suffered a dislocated nose after being assaulted by a laughing gas seller at Notting Hill Carnival - before going straight back to work, Scotland Yard said.
The officer was punched in the face after stopping the man selling nitrous oxide in Lonsdale Road on Monday afternoon, according to police.
But after being treated by medics for a dislocated nose, a split lip and a swollen knee, the police officer went straight back to his duties.
The man, who was allegedly caught with 400 laughing gas canisters, was arrested on suspicion of actual bodily harm.
The investigation into the nitrous oxide canisters was referred to council trading standards officers, police said.
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Scotland Yard said more than 3,500 canisters of laughing gas with a street value of over £17,000 were seized at the carnival in all.