Two police officers have been injured in what the home secretary has described as a "disgusting, violent attack".
The officers stopped to help an apparent assault victim when they were set upon by a group of bystanders.
Video on social media shows an officer pinned down on the ground and being kicked, while his female colleague is pushed when she tries to intervene.
Several men surround the Scotland Yard officers swearing, shouting and filming on their phones, with one even dancing and taking a selfie during the assault.
Both officers suffered minor injuries in the incident, in Frampton Park Road, Hackney, east London, at around 3.30pm on Wednesday, Scotland Yard said.
They had been flagged down by a member of the public who said they had been attacked and pointed them towards the suspects, a spokeswoman added.
The Metropolitan Police Federation, which represents rank-and-file officers, said attacks on officers seem to be increasing, adding: "Yet again this starkly shows the dangers @metpoliceuk face and the bravery they show each and every day keeping Londoners safe."
Chairman Ken Marsh said: "We are not society's punchbags. We have families we want to go home to at the end of every shift. But the dangers are stark. And seemingly escalating."
Home Secretary Priti Patel tweeted: "Sickening, shocking & disgraceful.
"My thoughts are with the outstanding @metpoliceuk officers who were subject to this disgusting violent attack."
John Apter, chairman of the Police Federation, said: "Disgusting behaviour from those who attack the officers, those who film what's going on and those who do nothing. They are cowards, the lot of them."
A Scotland Yard spokeswoman said two men, aged 20 and 38, have been arrested on suspicion of assaulting police. They remain in custody in an east London police station.
Mayor of London, Sadiq Khan, added: "I utterly condemn the disgraceful attack on two Metropolitan Police officers.
"These brave officers were doing their duty and assisting the public. We owe them a debt of gratitude and I wish them a full and speedy recovery.
"Attacks on our police will not be tolerated. I urge anyone with information to come forward to ensure the perpetrators of this disgusting attack feel the full force of the law."