Violent suspects could be released by police if officers do not get "backed up" by members of the public, a federation leader has warned.
Ken Marsh spoke out after a video of officers being attacked was shared widely on social media.
Several cars can be seen driving past the encounter without stopping.
Met Police Federation chairman Mr Marsh 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?"
However, Mr Marsh added this did not mean he was asking members of the public "to suddenly jump in to assist police officers, because we are highly trained in what we do".
The video, taken in Merton, south London, on Saturday, shows a man aim a flying kick at a female officer, who is left clutching her head just yards away from a passing bus.
Another male officer is dragged across the road as he tries to stop a second suspect from running away.
The male officer suffered cuts and the female officer was left with head injuries. Two other suspects are wanted by police.
A 20-year-old man has been charged with assault causing actual bodily harm and assaulting an officer.
A member of the public wearing a motorcycle helmet helped the male officer, but several cars went past without stopping.
Talking on BBC Radio 4's Today programme, Mr Marsh said: "If the public now think it's OK to stand and film and mock my colleagues...then we are in very dangerous grounds.
"What I'm trying to get across is the simple fact that society has changed so much lately that it seems to be OK [to be] more interested in mocking us and filming us."
He added: "We're going to come to a point where we're going to start pushing messages out to our colleagues: 'Risk-assess it dynamically and, if you think you can't detain a person, just let them go'."
A former Met officer, who left the force in 2014, said officers were attacked on a daily basis.
Mark - not his real name - told BBC 5 Live Breakfast: "I knew officers that were stabbed, punched, kicked, spat at [or] scratched so that they bled as a relatively regular occurrence."
"I think a lot of it goes massively unreported," he added.
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.
Statistics on 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.
Assistant commissioner Steve House of the Met Police 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.
"This training is substantial and delivered in accordance with national guidelines and we regularly review it to make sure it is fit for purpose.
"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."