Six people were injured in an acid attack at Westfield Shopping Center in Stratford, east London, on Saturday.
Panicked victims were heard screaming in pain and shouting, "I can't see," after the attack, eyewitnesses said.
Staff at a nearby Burger King said that one victim ran into the restaurant for help to wash the substance off his face.
"There were cuts around his eyes and he was trying to chuck water into them," the restaurant's assistant manager Hossen said.
A 15-year-old boy has been arrested on suspicion of grievous bodily harm. The Metropolitan Police confirmed that a group had attacked people with a "noxious substance," but clarified that this was not a random attack.
"I would like to be very clear concerning this incident," Chief Superintendent Ade Adelekan, Newham borough commander, said.
"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."
Emergency services arriving at the scene
The attack comes amid an alarming rise in the number of incidents involving acid in London, where more than 460 attacks involving corrosive substances took place in 2016.
In the first four months of 2017, London's Metropolitan Police recorded more than 100 attacks, many in East London.
The spate of attacks has been linked to a crackdown on other kinds of weapons such as knives and guns. A report sent to the Home Office last year by Conservative MP James Berry suggested preventative measures that would make it harder to obtain corrosive substances.
The report suggested restricting their sale to credit-card only purchases, or forcing manufacturers to reduce the corrosive content of household substances such as bleach.