Police are searching for a man who put an NHS worker in hospital after attacking him on a London bus.
CCTV footage shows a man punching the 63-year-old victim to the floor before stamping on his head on the 149 service near Seven Sisters station.
The victim was knocked unconscious and has no memory of the attack at about 21:45 BST on 23 August.
Police believe the assault began when the man avoided the attacker, who did not have his face covered.
No arrests have been made.
The victim works on the coronavirus track and trace system for the NHS, the Met said.
He suffered bruising and swelling to face, but left hospital the day after the attack.
PC Chris Bowman said: “We believe that this was all triggered by face masks.
“We think that the attacker, who had his face covering beneath his chin, took offence when the victim, who was fully covered, consciously moved away from him.
“When the attacker then follows the victim to the back of the bus, he can be seen pointing and gesturing at the victim’s face mask before he starts repeatedly punching and kicking him.
“This was a totally unwarranted violent assault and we urgently need to speak to this man.”
The attacker is described as a tall black man, aged approximately 23-25 years old, with an athletic build.