Staff challenging a suspected shoplifter were sprayed with a “noxious irritant”, Scotland Yard has said.
Three people were taken to hospital after the attack in Notting Hill, west London, on Wednesday.
Police said officers were called at about 2.10pm to a shop in Westbourne Grove after workers approached a man they believed was stealing.
“The man had then sprayed the staff with what police suspect to be a noxious irritant,” a Metropolitan Police spokeswoman said.
The London Ambulance Service (LAS) said one person was bleeding and taken to hospital as a priority, while two others were taken later on.
The Met spokeswoman added: “At this early stage their injuries are not thought to be life-threatening or changing.”
Detectives are working to establish what the substance is.
The London Fire Brigade (LFB) sent 10 firefighters in two engines to ensure the area was decontaminated, a spokeswoman said.
“Once they were satisfied it was safe they left the scene shortly after 3pm,” the LFB spokeswoman added.
An LAS spokeswoman said a hazardous area response team was also dispatched.
Officers are yet to make any arrests and inquiries continue.