The home secretary and the Police Federation have condemned a video which shows an officer on the ground apparently being kicked.
Video circulating on social media shows an officer struggling with a man in Frampton Park Road in Hackney, north London.
The footage was branded “sickening” by Priti Patel, while the federation said: “We are not society’s punch bags.”
Four people have since been arrested on suspicion of assault on police.
They include a 13-year-old boy and three men, aged 20, 32 and 34.
The officers, a man and a woman, suffered minor injuries but did not require hospital treatment, the Met Police said.
A member of the public, who had claimed to have been assaulted, flagged down the police on Wednesday afternoon.
In a statement, the Met said: “As an officer attempted to speak with those involved, one of the men resisted and a struggle ensued. A number of other people became involved whilst the officer was on the ground.”
Footage appears to show the officer trying to restrain a man but they then fall to the ground with the officer pinned underneath.
Passers-by began filming and taking selfies as the officer and the suspect grappled.
The female officer attempted to keep people away from the scene and also suffered injuries before more officers arrived and the first two arrests were made.
Tom Symonds, BBC Home Affairs correspondent
Two police officers, presumably awaiting backup, are having to fight for control, on a London pavement, surrounded by people, seemingly hostile.
As is so often the case, phones came out of pockets as the incident developed, so the video doesn’t show what happened in the minutes before.
That is crucial since officers have to justify the force they use.
Even before George Floyd’s death, any number of controversial arrests in recent years have demonstrated that a single video posted on social media can transform the public’s opinion of an incident, and therefore their views of the police.
This investigation may be able to rely on the officer’s own body-worn video cameras, increasingly vital in establishing the context.
But more generally, figures show the number of reported assaults on police officers have been rising steadily since 2014.
This, during a period when police numbers fell because of years of austerity. Fewer officers on the streets means fewer officers to respond, when their colleagues get into difficulty.
Ken Marsh, chairman of the Metropolitan Police Federation, said: “Yet again this starkly shows the dangers [officers] face and the bravery they show each and every day keeping Londoners safe.
“We are not society’s punch bags. We have families we want to go home to at the end of every shift, but the dangers are stark and seemingly escalating.”
Supt Martin Rolston said: “This incident, which was captured on someone’s mobile phone is truly shocking.
“My officers went to the assistance of a member of the public – who asked for their help – after stating that they had been assaulted.
“What happened next is a reminder of the risks our officers take whilst going about their duties.”
Home Secretary Priti Patel tweeted: “Sickening, shocking & disgraceful.
“My thoughts are with the outstanding officers who were subject to this disgusting violent attack.”
Mayor of London Sadiq Khan said: “I utterly condemn the disgraceful attack on two Metropolitan Police officers this afternoon.
“These brave officers were doing their duty and assisting the public. We owe them a debt of gratitude.”