Police were called to the Stratford Centre in the borough of Newham just before 8pm, a Metropolitan Police spokesman told The Independent.
The force said a “group of males” was reported as having sprayed “what is believed to be a noxious substance” and eight victims were injured in a number of locations.
One person has been arrested on suspicion of causing grievous bodily harm, it said.
Witnesses at the scene said an argument broke out among a group of people in the Stratford Centre, which is a much smaller shopping mall than its sprawling neighbour, the Westfield Centre.
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 fast food outlet’s toilets “to wash acid off his face”.
He said: “There were cuts around his eyes and he was trying to chuck water into them.”
Another witness, Zak Abdi, who shared footage from Stratford train station, told the Mirror Online he saw a substance thrown at a group of men “who looked like they were on their way to a club”.
“It didn’t hit just one person, it hit a crowd of people. One guy had been hit in the face, he kept shouting ‘I can’t see, I can’t see, I can’t see’.
“I think he has lost his vision. He kept shouting, it was a scary moment for everyone.”
Nate Higgins, 20, told the Independent he left Westfield shopping centre at about 8.30pm.
“I was in Westfield and I came out and saw all the police and ambulances.
“People weren’t panicking. It seemed like it was settled. But as I was leaving more and more ambulances and fire engines were showing up.
“There were more fire engines than I’ve ever seen in one place.”
Inside the Burger King toilets, water could reportedly be seen all over the floor, along with toilet tissue and medical gloves.
Other eyewitness accounts said a group of males roamed in and around the centre apparently picking off their victims, with several people posting footage of the aftermath on social media.
Victims appear to be receiving treatment in Stratford rail centre, which is directly opposite the shopping centre and close to the Olympic Park.
Panicked commuters can be seen fleeing the scene in some images, while footage shows police and security officers swarming around the CCTVs at the train station.
The incident is not being treated as terror-related, Scotland Yard said in a statement.
Paramedics and firefighters were also called, and a wide cordon was put in place.
Paul Gibson, LAS assistant director of operations, said: “We were called to the incident at 8.07pm and sent a number of resources including ambulance crews, paramedics in cars and members of our hazardous area response team.
“The first of our medics arrived within 10 minutes of the first emergency call and worked closely with our colleagues from the Metropolitan Police, British Transport Police and London Fire Brigade making sure patients received the medical help they needed as quickly as possible.
“We treated six patients in total and took three to London hospitals.”
Additional reporting by agencies