Six people were believed to be injured after being sprayed by a noxious substance in a shopping center in east London on Saturday, police said.
The injuries are believed to have occurred at different locations at the Westfield Shopping Center in Stratford, police said.
One man has been arrested, but police said they believed a group of men are responsible.
The incident is not believed to be terror-related, police said.
At least five patients were treated at the scene, and three of them were taken to hospital, Paul Gibson of the London Ambulance Service told BBC News.