Two people have been taken to hospital after an “unknown substance” with effects like CS gas was sprayed during a fight on Oxford Street.
A man, believed to be in his 20s, was arrested for affray after it is alleged he sprayed another man with the substance in Selfridges department store at about 13:40 BST on Thursday.
Paramedics treated 18 people for effects of the spray – two were taken to hospital for further treatment.
The Met said enquiries were continuing.
CS gas is used by UK police forces as a temporary incapacitant spray.
Exposure to CS gas may cause intense tear production, eye pain, chest tightness, coughing and sneezing.
When released, CS gas will disperse into the atmosphere in a matter of minutes.