A 22-year-old man has been arrested over the murder of Richard Mannington Bowes who died after being attacked during riots in Ealing.
Mr Bowes, 68, was set upon as he tried to stamp out a fire.
He suffered serious head injuries and died just before midnight last night, Scotland Yard said.
His next of kin have been informed and a post mortem examination will be held soon.
Detective Chief Inspector John McFarlane, of the Met's Homicide and Serious Crime Command, said: "This was a brutal incident that resulted in the senseless killing of an innocent man.
"I still need the assistance of the community who may have witnessed the attack to come forward and provide information or images they may have recorded on mobile devices.
"This information could be crucial in catching his killer."
Jim, the owner of Big Jim's Trims in Ealing, said rioters attacked Mr Bowes when he tried to put out a fire they had started in a supermarket bin near the Arcadia shopping centre.
Jim, 40, who has two hairdressing shops, on The Green and in Devonshire Road, said: "I went up to Spring Bridge Road to check on my other shop and saw him on the ground.
"The rioters had set a bin alight and then they jumped on him when he tried to put it out."
Scotland Yard said officers were in the area dealing with violent disorder when they became aware of Mr Bowes being attacked.
They managed to reach him to administer first aid despite missiles being rained at them, the Met added.