The attack happened while the 61-year-old victim was travelling on a route 56 bus through Essex Road, Islington, on January 3.
He was approached by a girl and two young men, and one of them began to harass him.
When the man tried to stand up and move away, the group began to punch and kick him.
The victim called out for help, and the three youths ran off leaving the man injured.
A Scotland Yard spokesperson said: “The victim was left with two black eyes and has been left very distressed as a result of the assault.”
On Thursday, police issued CCTV images of three people they are keen to trace in connection with the attack.
Any witnesses or anyone with any information is asked to call police on 101 or contact via Twitter @MetCC. Please quote CAD 4128/03JAN. To give information anonymously contact Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org.
