Twenty-two police officers have been injured and their vehicles damaged during clashes at an illegal street party in south-west London.
Two officers and two people at the party were taken to hospital following the “unlicensed music event” in Brixton on Wednesday night.
Four people were arrested and the Met described the attacks on police as “totally unacceptable”.
Downing Street said “violence against the police will not be tolerated”.
The Prime Minister’s official spokesman said the government would shortly be launching a consultation on doubling the maximum sentence for assaulting an emergency worker.
“We have been clear that anyone who assaults the police or any of our emergency service workers who keep us safe should feel the full force of the law,” the spokesman said.
A dispersal order had been put in place earlier in the day and police officers tried to encourage the crowd to leave following complaints from residents.
The event near Angell Town continued so more officers attended and the group became hostile, the Met said.
Footage on social media showed a number of police vehicles being smashed and officers being chased.
Another video also appears to show two men being attacked with a knife.
London Ambulance Service said two people were taken to hospital, with one needing treatment at a major trauma unit.
Four people were arrested for assault and public order offences. They remain in custody.
Michael, a local bus driver who did not want to give a his surname, said it was “very upsetting” the violence had happened in his area.
“The whole thing should have been shut down earlier,” he said.
Police later implemented a Section 60 order, which gives increased stop and search powers.
Ken Marsh, chairman of the Metropolitan Police Federation, said nearly 30 of his colleagues were “walking wounded”.
“No-one expects this level of violence and hostility to just erupt at the speed it does towards police. It’s horrendous,” he said.
At the scene
Katharine Carpenter, BBC London Home Affairs Correspondent
The day after the night before the only evidence on the Angell Town estate of what happened is some broken glass and plastic bottles.
People who’d been at the event tell me it all started peacefully with a barbeque and music.
But it “got out of hand” when people from other areas flooded in.
While condemning the violence young people told me there is “no relationship” with the police and the community officers they knew don’t come around anymore.
I met a mum who said she’s been campaigning for a new youth centre, “the kids don’t have anywhere to go”, she said.
Mr Marsh also raised concerns about large gatherings when lockdown is eased and pubs reopen on 4 July, saying “ultimately it always seems to fall on the police’s shoulders to deal with everything”.
“There is no place in society for disorder and wanting to cause violence and pain and grief towards public and police alike,” he said.
Home Secretary Priti Patel tweeted: “Just last weekend, the whole country came together to praise our heroic police officers for putting their own lives on the line to keep us safe.
“I’ll be picking up with the Met Commissioner immediately.”
Cdr Colin Wingrove said: “These gatherings are unlawful, as well as posing a risk to public health and against coronavirus restrictions.
“The violence shown towards officers is totally unacceptable and we will not tolerate it in any form.”