Footage of the incident posted on Twitter shows a man wielding a knife in front of shocked shoppers at the department store on Oxford Street.
Another man threw a mannequin onto the floor before two men started fighting.
The clip then shows the group making their way to another part of the store as security guards followed.
A spokesperson from the Met Police said the injured man had been stabbed in the leg and he is not in serious condition.
He has since been arrested on suspicion of affray and is being held in custody.
Seven more people have been arrested on suspicion of GBH including a 21-year-old man; a 20-year-old man; an 18-year-old man; a 20-year-old man; a 22-year-old man; a 26-year-old man and a 20-year-old woman.
All remain in custody at this time.
In a statement, the Met said: “Police were called at approximately 7.35pm on Saturday, 8 May to reports of a fight at a store in Oxford Street, W1.
“Officers attended the scene and found a male with a stab injury to the leg.
"He has been taken to hospital for treatment – his condition is not thought to be serious.
“He is one of seven people arrested as part of this investigation for offences including affray.”
Detectives from CID based at Westminster investigate and ask anyone who witnessed this incident, or anyone who has footage, to contact them on 101 or tweet @MetCC and quote CAD 6085/8May.
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