Six people are believed to have been injured in Stratford, east London, after reports they had been sprayed with a “noxious substance”.
Police were called to the Westfield Shopping Centre just before 20:00 BST. A man has been arrested on suspicion of grievous bodily harm.
The Metropolitan Police said there had been reports of a group of males spraying people with the substance.
Ambulance and fire services are also at the scene.
Those reported injured are believed to be in a number of different locations.
The incident is not believed to be terror related.
A police spokeswoman said a cordon was in place at the shopping complex.