Children are among nine people injured after a "corrosive substance" was thrown in Clapham in south London.
London Fire Brigade said they were "called to a chemical incident on Lessar Avenue" and provided emergency care to a woman and two children.
London Ambulance Service said 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," it added.
Police said three officers were among those hurt but their injuries are believed to be minor.
A helicopter is now involved in a manhunt to catch the person responsible.
"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 tests were ongoing but that the substance was believed to be "corrosive".
London Assembly member Marina Ahmad said on X that a man had thrown acid at people in a car following a crash.
The incident happened shortly before 7.30pm on Lessar Avenue, a street running from the south of Clapham Common.
Pictures show a white hatchback with the doors open in the middle of the road.
This breaking news story is being updated and more details will be published shortly.
Please refresh the page for the fullest version.
You can receive Breaking News alerts on a smartphone or tablet via the Sky News App. You can also follow @SkyNews on X or subscribe to our YouTube channel to keep up with the latest news.