Two teenagers have been arrested after acid was thrown in people's faces in five attacks over one night in London.
Two moped riders attacked people in a 90-minute spree in Islington, Stoke Newington and Hackney on Thursday, stealing mopeds in two of the attacks.
An eyewitness said he heard a victim, who he believed was a delivery driver, "screaming in pain". One victim suffered "life-changing injuries".
Police are looking at whether moped theft was the motive for the attacks.
Officers said they were linking the attacks and boys aged 15 and 16 have been arrested on suspicion of robbery and causing grievous bodily harm.
Delivery services Deliveroo and UberEATS have confirmed two of the victims were couriers working for the firms.
The attacks happened amid rising concern about the number of assaults involving corrosive substances in London.
Since 2010, there have been more than 1,800 reports of attacks involving corrosive fluids in the capital. Last year, it was used in 458 crimes, compared to 261 in 2015, according to Met Police figures.
Hackney resident Jon Moody said he was watching TV when he heard screaming and ran to the window.
"I heard a high-pitched scream but thought it was the boys playing football... I heard more shouting and ran to my window," he said.
"I could see a man in serious distress, he was screaming in pain.
"There were only two police officers with the victim, they took out two large water canisters and poured it over him."
He said he believed the victim was a delivery driver and about 20 fellow delivery drivers turned up at the scene.
The Hackney Gazette last week reported many delivery drivers are refusing to work in some areas after 21:30 BST because of robbery fears.
Takeaway delivery firm Deliveroo emailed drivers saying it was working with the Met Police and urged its staff to report any information about the attacks.
The email said the firm was "truly shocked" about what had happened.
The assaults happened amid increasing concern about the sharp rise in acid attacks in London.
Met Commissioner Cressida Dick said the growing trend of victims being doused with corrosive liquids was concerning.
Ms Dick told LBC Thursday night's attacks were "completely barbaric".
"The acid can cause horrendous injuries," she said.
"The ones last night involved a series of robberies we believe are linked - I am glad to see we have arrested somebody."
A Met spokesman said one line of inquiry detectives would be pursuing was whether the attackers were targeting moped riders to steal their bikes.
The 16-year-old boy was arrested in Kingsbury Road, north-west London, early on Friday, while the 15-year-old was arrested in Stoke Newington several hours later.
The attacks began at 22:25 BST on Thursday in Hackney Road.
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A 32-year-old man on a moped was left with facial injuries after another moped, with two male riders, pulled up alongside him and threw a corrosive substance in his face.
One of the men stole his moped and the other drove away on the vehicle they arrived on.
The Met said it was awaiting an update on the extent of the victim's injuries. Inquiries are ongoing.
Thursday night attacks timeline
- 22:25 BST: A 32-year-old man on a moped in Hackney Road is left with facial injuries after a moped, with two male riders, pulls up alongside and attacks him. They also steal his moped.
- 22:49 BST: A 44-year-old man has a corrosive substance thrown in his face by two men on a moped in Islington.
- 23:05 BST: A corrosive substance is reported to have been thrown in the face of a 52-year-old man by two men on a moped in Shoreditch High Street. The victim is taken to hospital.
- 23:18 BST: Police receive reports of a robbery in Cazenove Road, Stoke Newington, where a corrosive substance is thrown. A 24-year-old man suffers facial injuries described as "life changing".
- 23:37 BST: A 33-year-old man on a moped in traffic in Chatsworth Road, Clapton, has liquid sprayed in his face and his vehicle stolen.
Assaults involving corrosive substances have more than doubled in England since 2012, with the number of acid attacks in the capital showing the most dramatic rise in recent years.
The Met's own figures show there were 261 acid attacks in 2015, rising to 458 last year.
So far this year - excluding Thursday night - the Met has recorded 119 such attacks.
A man appeared in court earlier this week in connection with a separate attack on cousins Resham Khan and Jameel Muhktar, who had acid thrown at them through a car window in Beckton, east London.
Shadow Home Secretary and Stoke Newington MP Dianne Abbott responded to news of the attacks, tweeting: "More terrible acid attacks, Why would you scar someone for life just to steal a moped."
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Labour MP for East Ham Stephen Timms has tabled an adjournment debate for Monday in the House of Commons on the rise in the number of acid attacks.
About a third of last year's acid attacks in the capital took place in the London borough of Newham, which is in his constituency.
Mr Timms told BBC Radio 4's Today programme he was "most concerned about sulphuric acid" and that carrying a bottle without justification should be treated as an offence, like carrying a knife.
"We could certainly come up with arrangements that would allow people to use sulphuric acid in the normal way, perhaps with the benefit of a licence.
"But simply walking around the street with a bottle of sulphuric acid, that should be an offence," he said.
A Downing Street spokesman said the prime minister viewed acid attacks as "horrific".
"We are working with the police to see what more we could do. The prime minister's view is that the use of acid in this way is horrific."
Home Office minister Sarah Newton told BBC Radio 5 live Breakfast the government was considering tighter controls on some chemicals in response to the acid attacks in East London and elsewhere.
But she said regulation would be difficult, as "these chemicals are under everyone's kitchen sinks".
She said it was clear acid was being used "as a weapon" and work had been commissioned "to understand the motivation" of people who use it to injure others.
She also said the government was examining sentencing for those who use acid to injure people.