Passers-by should only help officers struggling with violent suspects "if safe to do so", a police chief said.
Metropolitan Police Assistant Commissioner Sir Steve House spoke out after warnings that suspects could go free if officers were not "backed up" by members of the public.
It follows footage of two officers being attacked as they tried to make an arrest in south London.
He said said the public do not have protective equipment like police do.
The video, which has been widely shared on social media, shows one officer being dragged around in the road while the other took a flying kick to the chest and was left yards away from a passing bus.
Sir Steve said: "Whilst officers should never expect to be attacked as part of their job, a core part of officer safety training is ensuring they know how to respond to volatile situations.
"Officers are also issued with personal protective equipment to help protect them and the public.
"I am mindful that members of the public do not have access to such items and, while any officer would be grateful for the public to assist them with a difficult arrest, they should only intervene if they can safely do so."
He said suspects trying to flee arrest was "not a new phenomenon" and that officers in London knew there was "overwhelming support for the work we do from the public".
The male officer who was attacked suffered cuts and the female officer was left with head injuries.
A 20-year-old man has been charged with assault causing actual bodily harm and assaulting an officer.
Two other suspects are wanted by police.
The footage, which was posted on Twitter, showed several cars going past without stopping but a member of the public wearing a motorcycle helmet helped the male officer.
The footage has sparked a debate after Met Police Federation chairman Ken Marsh warned officers might be forced to let violent suspects go.
He said: "We don't come to work to get assaulted, and if we're not going to be backed up... then what is the point?"
Reality Check: Are assaults on police increasing?
Assaulting an on-duty police officer is an individual offence which can result in up to six months in custody.
In 2017-18, there were 26,295 assaults on police officers in England and Wales.
The Home Office comes to this figure by combining cases of assault with and without injury.
The overall number of police officers is 122,000.
There has been a general upward trend since the year ending March 2015, about the same time violent and knife crime began increasing after years of decline.
Figures for assaults on police with injury were only added to crime statistics this year, but those without injury go back to the early 2000s when the numbers were regularly about 20,000 a year.