Violent suspects could be released by police if officers do not get support from members of the public, a federation leader has warned.
Ken Marsh spoke out after a video of officers being savagely 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?”
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.
A 20-year-old man has been charged with assault causing actual bodily harm and assaulting an officer.
‘Not worth it’
A member of the public wearing a motorcycle helmet helped the male officer, but several cars went past without stopping.
Mr Marsh said: “Are we now in a society where, if we think we can’t detain somebody, we just let them go? It’s just not worth it.
“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’.”
The male officer suffered cuts and the female officer was left with head injuries.
Two other suspects are wanted by police.
Home Office guidance says officers must not seek or encourage members of the public to assist in any way in a pursuit and/or arrest.
“If any member of the public chooses to join a pursuit you must use your judgement to decide whether to instruct the public to desist (for example if you are aware the offender is armed or has a history of violence) and whether to call off the pursuit”, the guidance adds.
The Met has been asked to comment. The BBC has not yet been able to source the video.