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".