Three people have been injured after an 'unidentified substance' was delivered to the Feng Sushi restaurant in Stoney Street, close to Borough Market.
Met Police officers, the London Ambulance Service and the London Fire Brigade sent crews to the scene.
The restaurant was evacuated as a precaution.
The substance was later identified as "naturally occurring".
Three people were treated at the scene for minor injuries as the result of an irritant and did not require hospital treatment.
"At this early stage this is not being treated as a terror related incident," said a Met spokesman.