Police officers were greeted with "bricks and other missiles" as they tried to break up an unlicensed music event in West London in the early hours of Saturday morning.
Seven officers sustained injuries, though their scale was not revealed by London's MET Police.
"Officers attempted to engage the crowd in order to get them to leave the location but the group became hostile", the authorities said, adding that they were all dispersed at around 1.15 am local time.
No arrests have been made so far.
The incident is the latest in a series of illegal gatherings in the British capital over the past couple of weeks that have descended into violence.
Last week, 22 police officers were slightly injured after clashes at a street party in the Brixton district. Another gathering in Notting Hill in west London also led to violence.
England has reopened pubs and restaurants on Saturday after three and a half months - exception made for the town Leicester, which is undergoing further restrictions following an infection spike - but gatherings are still limited to a maximum of six people.
In addition to the reopening of much of the hospitality sector, couples can tie the knot once again, people can go to their local cinema and also finally get a haircut.
Boris Johnson on Friday urged people not to "blow" efforts made to tackle the pandemic as the country is "not out of the woods yet".
He also confirmed the government will opt for local lockdowns rather than nationwide restrictions in case of new outbreaks in the coming months.
The UK remains Europe's worst-hit country in the COVID-19 crisis, accounting for over 44,000 fatalities and more than 285,000 cases.
Authorities on Saturday echoed the prime minister's appeal for caution urging people to follow restrictions and maintain social-distancing for their "own health and safety".