A volunteer with a community security group, Shomrim, suffered a fractured skull after an incident in Golders Green in the early hours of last Thursday morning.
The father-of-three had responded alongside colleagues after a number of Orthodox Jews reported being hit by eggs thrown from a car. The members of the security group tracked the vehicle and surrounded it until police officers arrived.
It is understood that the volunteer, who is in his 40s, was subsequently knocked unconscious and suffered serious injuries after falling from the car's bonnet shortly before the vehicle sped from the scene.
Eyewitnesses claim that there was poor co-operation between the police officers who attended the scene and the Shomrim members and the volunteers from the Hatzola ambulance service who had rushed to treat the man.
But there are also claims that the volunteer, who was recovering in hospital this week, had climbed onto the car and that his actions had been "idiotic".
However, Shomrim chairman Sheldon Bodner denied the volunteer was at fault.
"While protecting the community in this incident our volunteers followed full protocol until the arrival of police," he said.
"We feel the direct result of the actions taken by police seriously jeopardised the safety of our members."
Shomrim has regularly assisted police in tackling criminal activity in north-west London.
Barnet police commander Chief Superintendent Adrian Usher said the incident was still under investigation. He rejected claims his officers were to blame.
An 18-year-old man from Finchley was arrested later on Thursday and questioned on suspicion of attempted murder. He has been bailed to return to a police station next month.