Six people have been injured after a noxious substance was thrown at a shopping centre.
A 15-year-old boy has been arrested on suspicion of causing grievous bodily harm after the incident at the Stratford Centre, opposite Westfield, in east London.
Three of the victims needed hospital treatment after being hit by the substance during an altercation involving two groups of males shortly before 8pm on Saturday.
Onlookers described seeing one young man screaming in pain as friends shouted "it is an acid attack, his skin is burning", while others rushed to clean their wounds with water.
Their injuries were not said to be life threatening or life changing.
A man who gave his name as Hossen, 28, an assistant manager at Burger King, said he saw a victim and his friend, a local homeless man, run into the fast food restaurant's toilets to "wash acid off his face".
He said: "There were cuts around his eyes and he was trying to chuck water into them."
Members of the London Ambulance Service (LAS) hazardous area response team also attended the scene.
Paul Gibson, assistant director of operations at the London Ambulance Service, said medics arrived within 10 minutes of the first emergency call and provided first aid with help from the police and fire crews.
He said: "We treated six patients in total and took three to London hospitals."
Chief Superintendent Ade Adelekan, Newham borough commander, said: "I would like to be very clear concerning this incident.
"What initially may have been perceived as a number of random attacks has, on closer inspection, been found to be one incident involving two groups of males."