Officers were called at 2.22pm on Wednesday afternoon to reports of a "noxious substance" being spilled at a commercial venue, London's Metropolitan police said.
It prompted them to close down Brompton Road in Knightsbridge.
Emergency services later confirmed the spillage came from restaurant where 15 members of staff had evacuated the building. Another 27 were evacuated from a neighbouring block of flats.
A spokeswoman for London Ambulance Service said: "We dispatched our hazardous area response team (HART), an ambulance crew, a medic in a car and an incident response officer to the scene.
"We treated a person at the scene and took them to hospital."
Police, fire and ambulance crews were scrambled to the scene following the spill, which the has confirmed is not being treated as terror related.
It is believed the incident took place at a Kuwaiti restaurant - however the nature of the spilled chemical remains unclear.
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