Two men on a moped carried out five acids attacks during a spree across London which lasted less than 90 minutes, police said.
The Metropolitan Police said one victim had been left with “life-changing” injuries after being doused on Thursday night in the east of the capital.
The assaults appeared to be linked and two involved victims having their mopeds stolen, they added.
A male teenager was later arrested on suspicion of grievous bodily harm and robbery and has been taken to an east London police station.
At the start of the rampage, a 32-year-old moped driver had been approached by the pair as he drove towards the Hackney Road junction with Queensbridge Road.
London Ambulance Service said they were called to the first attack on Hackney Road at 10.16pm, while police were alerted to the night’s final assault at 11.37pm.
In the initial encounter, the two male suspects had tossed the noxious substance into his face before one of them jumped on to the victim’s vehicle and drove away.
Police said the man had gone to an east London hospital and they were awaiting an update on his injuries.
Little more than 20 minutes later, at around 10.50pm, another victim had been sprayed with searing liquid by the pair on the Upper Street junction with Highbury Corner, Islington.
The victim was taken to hospital in north London.
Then at around 11.05pm, the fast-moving attackers swooped on a man in Shoreditch High Street, tossing the substance in his face.
His injuries were not life-threatening, police said.
Within 15 minutes, they appeared to have struck again, launching their corrosive load at a man on Cazenove Road and causing “life-changing” facial injuries.
The final assault of the night was reported to police at 11.37pm, when another man was confronted as he sat on his moped in traffic on Chatsworth Road.
After again spraying the liquid in a victim’s face, the moped was stolen and both attackers fled.
Hazardous area response units were sent by the ambulance service to four of the scenes, while the victim of the Chatsworth Road attack took himself to hospital.
Witness Sarah Cobbold looked out from her flat near the Hackney Road attack to see police pouring huge bottles of water over the victim on her doorstep.
The 29-year-old sub-editor had been standing at the same spot just minutes earlier.
She said: “It was just outside my flat, the victim was literally stood on my doorstep.
“It was probably about 10.30pm, I saw some reports that it happened at 10.25pm so by 10.30pm there were a couple of police cars and an ambulance there.
“After about 10 minutes I still saw the flashing and went out on my balcony and looked over the roof to see what is going on.
“Police had cordoned off the little area around the pavement and there was just a guy standing on my doorway and they were pouring huge, five-litre bottles of water over his head.
“He was standing, he seemed to be OK. After probably about 10-15 minutes he managed to walk unaided into the ambulance. I hope that means he is OK; the ambulance certainly didn’t rush off with sirens.”
As she watched, around 25 moped riders arrived - many appearing to be from fast-food delivery companies such as UberEats and Deliveroo.
“I’m guessing maybe they had heard what happened and came down because they can’t have all been driving round together,” she said.
She added: “I had thought someone must have chucked petrol or acid on him or something because they were covering him in water, but I have never seen that reaction to an attack, I thought maybe there had been an accident.
“I had literally just walked in the house, I was stood where he was 10 minutes earlier.
“I don’t think I would have been attacked but it could have been quite easy to have been caught in the crossfire here or at the other locations.”
Police cordons had been lifted by the morning, but the street remained strewn with empty plastic bottles, Ms Cobbold said.
The Met Police said in a statement: “Inquiries are ongoing and officers from Hackney CID are investigating.
“Any witnesses, anyone with information or in possession of footage of these incidents should contact police on 101 or tweet @MetCC. To remain anonymous you can call Crimestoppers on 0800 555 111.”