At least 22 police officers were injured after a street party in south London descended into violent clashes.
Police were called to Brixton, Lambeth on Wednesday evening after reports of a large unlicensed music event in the street.
In a statement, Metropolitan police say the crowd "became hostile towards officers" after authorities tried to encourage people to leave.
Videos on social media show individuals brandishing weapons and throwing bottles at armed officers. Other footage from the scene shows men jumping on a police car and smashing in the windows as officers flee.
Although no officers were seriously injured in the clashes, two did require hospital treatment. A small number of police cars were also damaged in the incident.
Four individuals have been arrested for assault and public order offences.
London Mayor Sadiq Khan said large gatherings during the pandemic are "deeply irresponsible" and are a risk to life.
"Violence against the police will not be tolerated."
UK Home Secretary Priti Patel also described the images as "utterly vile scenes'' on Twitter.
“Our officers work incredibly hard to engage and explain the public health message and regulations to prevent such gatherings occurring," add BCU Commander Colin Wingrove.
“Our role is to protect the public and ensure guidelines are adhered to in order to prevent the spread of COVID-19 protecting the NHS and save lives".
An investigation has been launched into the incident, and officers are conducting "reassurance patrols" in the area on Thursday.
Police and local authorities in Bournemouth have also declared a "major incident" after thousands of people descended on beaches on the UK's south coast on Wednesday.
"A multi-agency emergency response has now been activated to coordinate resources across the area to tackle the issues," said Bournemouth, Christchurch and Poole (BCP) Council in a statement.
“The irresponsible behaviour and actions of so many people is just shocking," said council leader, Vikki Slade.
"Our services are stretched to the absolute hilt trying to keep everyone safe."
Under coronavirus lockdown restrictions, groups are limited to six people, but warmer weather in recent weeks has seen far larger gatherings.
The UK government has also warned that groups must continue to socially distance from people outside their household.
England will reopen pubs, restaurants, hairdressers and cinemas as part of a major easing out of lockdown on July 4.