Reports of an assault on a group of people were received shortly after 8.10am this morning (December 7).
Emergency services, including armed police, were dispatched to a multi-storey car park at Terminal 3.
At the scene, 21 people were treated for injuries.
A three-year-old girl was among patients.
Five people were taken to hospital with injuries not thought to be life changing or threatening.
A 31-year-old man was arrested on suspicion of assault within nine minutes of the report coming in.
He remains in custody and enquiries remain ongoing to locate further suspects.
New detail has been shared by the Metropolitan Police, alleging a substance believed to be pepper spray was used in a robbery inside a lift.
Cmndr Peter Stevens said: “At this stage, it’s understood that a woman was robbed of her suitcase by a group of four men, who sprayed a substance believed to be pepper spray in her direction.
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“This occurred within a car park lift, with those in the lift and surrounding area affected by the spray.
“Our officers are working to determine the full circumstances around what happened but we do believe this to be an isolated incident with those directly involved known to each other.”
Anyone with information about the incident is asked to call 101, quoting CAD 1803 7 DEC.
