The Metropolitan Police has shut down three illegal raves as part of efforts to tackle anti-social behaviour in the capital.
Hundreds of people had attended the events and some became hostile towards officers as they began to disperse crowds, the Met said.
Officers attended and found 300 youths at a community centre.
The Met said a number of fights broke out as people began to leave and fireworks and other projectiles were thrown, and back-up officers had to be called.
Crowds were dispersed at 1.20am on Saturday and one officer was treated for injuries.
The Met was later made aware of another unlicensed music event in railway arches on Corbetts Lane in Southwark.
About 150 people were dispersed from the area and the sound system was seized, with the organiser fined for licensing offences.
Across the capital officers also attended an address on Birkbeck Street in Bethnal Green, having established that another unlicensed event was planned.
When police arrived 30 people were present and quickly left.
Chief Inspector Jack May-Robinson said: “Whilst we appreciate that a lot of people will be celebrating Halloween this weekend with house parties, there are a number of people who will choose to host large music events.
“These gatherings blight the lives of local residents.
“They often attract anti-social behaviour and can be dangerous environments for those attending them.
“Where we identify an event, we work hard to ensure that appropriate action is taken and equipment is seized to prevent attendees moving to another location.
“Extra patrols are taking place all over London to maintain public safety.
“I would urge anyone with concerns to contact police so that we can interrupt these UMEs (unlicensed music events) before they escalate.”
