Children and police officers are among nine people injured after a “corrosive substance” was thrown in Clapham in south London.
The fire brigade said they were called to a “chemical incident” and gave emergency care to a woman and two children.
London Assembly member Marina Ahmad posted on X that a man had thrown acid at people in a car following a crash.
Pictures show a hatchback with the doors open in the middle of the road and a helicopter is now involved in a manhunt to catch the person responsible.
A total of nine people were treated at the scene, with five taken to a major trauma centre.
“Three patients were taken to a local hospital and the other was discharged at the scene,” added London Ambulance Service.
Three police were among the injured, but their injuries are believed to be minor.
“We are drawing on resources from across the Met to apprehend this individual and work is ongoing to determine what has led to this awful incident,” said Detective Superintendent Alexander Castle.
He said the substance was believed to be “corrosive” but that tests were ongoing.
The incident happened shortly before 7.30pm on Lessar Avenue, a street running from the south of Clapham Common.