A 17-year-old boy has been arrested after a "noxious liquid" was thrown over four men near a shopping centre.
It happened on a footbridge linking Chafford Hundred station to Intu Lakeside in Essex on Monday.
The teenager was taken into custody on suspicion of causing grievous bodily harm.
The victims, in their 20s, were taken to hospital, while two police officers were also treated at the scene.
Witnesses reported seeing fire crews "hosing down" the victims at 20:10 GMT.
Essex Police said the injured men reported irritation to the eyes but their injuries were not thought to be life-threatening.
Officers are still looking for two more people who were thought to be involved in the disturbance.
Det Ch Insp David Browning said patrols would be stepped up in the area while police investigate.
He added: "Incidents of this nature are thankfully rare and the victims are fortunately not severely injured."