A fifteen-year-old boy is being held by police after six people were injured in an alleged acid attack in east London.
Three victims required hospital treatment after being hit with the substance during an "altercation" between two groups of males at the Stratford Centre on Saturday evening.
Witnesses described scenes of panic in the immediate aftermath.
Officers arrested the teenager later in the evening on suspicion of causing grievous bodily harm.
Scotland Yard said the two groups dispersed after after the "noxious substance" was thrown.
A young male could be heard screaming in pain as friends shouted "it is an acid attack, he is burning", according to witnesses.
Both ambulance and fire services attended.
Video footage showed at least one of the injured being treated at the scene.
Imran Tahir Rizvi said he overheard the friends of one victim, aged 18 or 19, shouting about an "acid attack", then saw a young man lying on the floor screaming of a burning pain.
"(They were) screaming and shouting for help as he was feeling a burning sensation on his skin," Mr Rizvi said.
"His fellows were shouting at police for something. Initially people thought like it was a fight. But the guys (with) the victim started shouting 'it is an acid attack, he is burning'."
A man who gave his name as Hossen, 28, a Burger King assistant manager, said he saw a victim and his friend, a known local homeless man, run into the restaurant's bathroom "to wash acid off his face".
"There were cuts around his eyes and he was trying to chuck water into them," he said.
Members of the London Ambulance Service hazardous area response team raced to the scene after the alarm was raised.
Three patients were treated at the scene and three were taken to hospital.
A Scotland Yard spokesman said: "During this incident a noxious substance was thrown.
"When the liquid was thrown, both groups ran from the scene."
Newham borough commander Ade Adelekan 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."