A man was sprayed in the face with fuel and then set alight when an argument broke out on the top deck of a bus.
Police have appealed for help to identify the man
The victim, in his late 20s, was on the N16 travelling along Edgware High Street, in north London, when a row broke out at about 2.30am on Saturday.
The noise woke up a sleeping man who also started arguing with the men. He then sprayed fuel in one man's face and set him alight, police said.
The victim is being treated for severe burns.
Police have released CCTV images of the attack.
Det Con Keith Melton said: "Whatever the circumstances of the initial disturbance, and the actions of the original group of males, there can be no justification for spraying petrol into a person's face and setting light to it."
They described the suspect as black, about 18 years old and 5ft 6in tall.
He was wearing a dark hooded top, headphones and a peak cap.