A 68-year-old man remains in a critical condition in hospital after suffering serious head injuries in London during the riots this week.
He was set upon as he tried to stamp out a fire on Monday night.
Scotland Yard on Wednesday said the man was attacked at about 10.45pm on a road in Ealing, west London, after remonstrating with teenagers who were setting fire to two industrial bins outside a shopping centre.
Officers who went to the aid of the victim, thought to be a local man, were pelted with missiles.
Detective Chief Inspector John McFarlane said: "I need the assistance of the community who may have witnessed the attack on this innocent man, to come forward and provide information or images they may have recorded on mobile devices.
"This information could be crucial in bringing those responsible for this terrible crime to justice."
He said the victim was "violently assaulted and knocked to the ground" as he tried to extinguish the fire, and added: "Police officers who were under attack by the mob drove them away and pulled the victim to relative safety and rendered first aid with the assistance of members of the public in an attempt to save his life.
"Again I urge these people to speak to us as they may have valuable information."
Police have released CCTV footage of a man suspected of carrying out the attack. Insp McFarlane said: "Through CCTV we have identified a strong suspect. I know that on seeing these images of him people will be able to identify him.
"If you know who this man is please contact my incident room.
If you are this man in the CCTV, do the decent thing and give yourself up."
A still image of a potential witness has also been released by the force.
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