Footage posted on social media showed scores of young people in a standoff with officers in riot equipment outside a block of flats in Harrow Road on Friday evening.
In the video, projectiles are seen being thrown at the line of officers, as one swings his baton at a group standing under a gazebo.
Another video appears to show officers being prevented from entering the melee by revellers. A man shouts: “You’re not coming in.”
It comes hours after Metropolitan Police commissioner Cressida Dick condemned attacks on officers sent to break up illegal street parties as “completely, utterly unacceptable”.
The UK’s most senior officer said the police had a “duty” to disperse large gatherings during the coronavirus crisis.
On Wednesday 22 police officers were injured when an unlicensed street party in Brixton descended into violence.
The following night officers sent to break up another gathering in Notting Hill were pelted with objects as they tried to disperse the crowd.
But as London basked in the tail-end of this week’s heatwave, dozens of undeterred revellers descended on Harrow Road from about 3pm on Friday.
One Twitter user wrote: “3rd avenue rammed with people. Looks like party vibes. Smh
“I had to turn back onto harrow road to get home. People think dis ah carnival. Joketing”
A Scotland Yard spokesman said: “Police are responding to an unlicensed music event on Harrow Road, W9.
“Officers were called to the location around 3pm following concerns from residents.
“Officers engaged with the attendees, who refused to leave the location. The police helicopter is also assisting.
“Following engagement within the local community we are hopeful that crowds are dispersing. No arrests at this stage.”
“A section 60 has been authorised in the vicinity until 6am on Saturday, 27 June.
Officers will remain in the area in order to provide reassurance.”