Two children, their mother and three 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 her two children.
Three members of the public who went to help are understood to be among those hurt, as well as three police officers.
Pictures show a car with the doors open in the middle of the road and a helicopter is now involved in a manhunt to catch the person responsible.
The incident happened shortly before 7.30pm on Lessar Avenue, a street running from the south of Clapham Common.
An eyewitness, who asked to stay anonymous, told Sky News: “There was a man in a white car with two children. It looked like he tried to run over a woman. They had been fighting.
“One girl in the back had her hand on the window – you can see the mark on the car.
“He grabbed one of the two children and violently grabbed them and slammed them on the floor.
“The lady then shouted ‘My eyes! My eyes! Call the police, my eyes!’
“Then I saw him run off. It was all so traumatising.”
Another witness described how he tried to wash the chemical from the woman’s face.
“It was horrific,” he told Sky News. “First a man grabbed his kid – a girl aged two or three from a white car. He threw her onto the floor – twice! It was the worst thing I’ve ever seen.
“The man then threw something at the woman. She was screaming: ‘My eyes! My eyes!’
“I then ran into my house, grabbed a water bottle and threw water on her eyes. Her lips were black. Her skin looked burnt.
“I then chased the man halfway down the street. I was wearing my slippers so didn’t get very far.”
The witness’s partner – a woman in her 20s – claims droplets of the substance also fell onto her.
She said: “I’ve just come back from hospital. We were both there.
“I was also with one of the children he threw to the ground. Her responsibility fell onto me.”
A total of nine people were treated at the scene, with five taken to a major trauma centre, according to the ambulance service.
Three more were taken to a local hospital and one was discharged at the scene.
“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.
DS Castle added that the injuries to the police officers didn’t appear serious.
Local councillors Alison Inglis-Jones and Ben Curtis thanked emergency services for rapidly cordoning off the scene and said they were “in total disbelief and shock at what has happened”.