A police officer suffered a head injury while trying to break up a 200-strong street party in east London.
The large group gathered on the streets in Ilford to celebrate ahead of the religious festival of Eid.
Met Police descended the scene to disperse the crowd after receiving reports from worried locals.
As the group were to head home, a fight erupted near Rutland Road and the officers rushed to break up the altercation between two groups of people.
Video emerged on social media captures men trading blows as baton-wielding cops arrest one suspect during the brawl at around midnight on Friday.
Officers can be heard yelling "get back" while trying to detain one man, who stumbles to the ground.
One police officer was injured during the chaos
The tape was captioned: "No place like Ilford, no place like home, this is actually the ghetto."
One man was arrested on suspicion of a public order offence and assault of an emergency worker.
Chief Superintendent Stephen Clayman of the Met's East Area Command said: "While we accept that celebrations take place, and we have no desire to spoil them for the community, if they descend into violence we absolutely will step in.
"A number of calls were received last night from concerned members of the public and while we appreciate this is a time of excitement and celebration for some, the sight and sound of such a large group was concerning and even distressing for others.
"That this size of gathering has taken place during a pandemic is concerning.
"I remind all of our communities to respect each other and ensure that the government guidelines around social distancing are followed."
He added that one of his officers has had to return home to his family with a head injury.
"This is beyond unacceptable and I would like to be very clear that anyone assaulting an officer in the course of his or her duty will be arrested and placed before the courts," he said.
The incident happened after the government announced the bombshell coronavirus rules with new lockdown restrictions in parts of northern England due to a recent spike of Covid-19 cases.
Health Secretary Matt Hancock said the stricter measures are being enforced because of "households gathering and not abiding by the social distancing rules".