Eight 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 Stratford Centre, opposite Westfield, 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.
Witnesses at the scene said an argument had broken out among a group of people.
A man who gave his name as Hossen, an assistant manager at Burger King, said a victim ran into the fast food chain to “wash acid off his face”.
The 28-year-old added: “There were cuts around his eyes and he was trying to chuck water into them.”