Two men on a moped have carried out five acid attacks during a 90-minute spree across London, leaving one victim with "life-changing" injuries.
The assaults appeared to be linked, and two involved victims having their mopeds stolen, they added.
A teenager was later arrested on suspicion of grievous bodily harm and robbery and been taken to an east London police station.
At the start of the rampage on Thursday night, a 32-year-old moped rider has a noxious substance tossed into his face and his moped stolen.
Soon another three other victims were doused with searing liquid by the pair, with one suffering "life-changing" facial injuries.
A fifth assault involving a man on his moped was reported to police just before midnight.
The Met Police said in a statement that inquiries are continuing.
The string of assaults comes just days after a man appeared in court accused of throwing acid at an aspiring model and her cousin.