POLICE arrested nine people - and at least four others were taken to hospital - during the illegal rave in East Croydon.
Thousands of people attended the event at the former Royal Mail delivery office in Cherry Orchard Road last night (Saturday).
Missiles were thrown at the police - including fire extinguishers - and a police vehicle was damaged.
Two people were detained for violent disorder and a further seven arrests were made for various other offences, including drugs and money laundering.
The Metropolitan Police said it is treating the area as a crime scene and expects to make further arrests in the coming days.
London Ambulance Service said paramedics had taken seven people to hospital, one for a head injury, another for a hand injury and others who had been taken unwell.
Police on a routine patrol noticed large numbers of people inside the building at around 9.40pm.
A spokesman said: "Additional officers were called and cordons were implemented to prevent further people from gaining access to the building and surrounding area.
"A number of people in the crowd became violent and missiles were thrown. The window of a police vehicle was smashed."
At that point two people were arrested for violent disorder. They remain in custody at a south London police station.
Police then allowed the event to continue but remained at the scene to prevent "dangerously large" numbers of people from entering the delivery office.
The premises suffered extensive damage during the rave, with more than a dozen windows broken and furniture and fire extinguishers thrown from the building.
At around 8.30am, police entered and began to clear the venue.
The cordon was kept in place this morning as the property was secured.
Tram services through East Croydon were delayed as a result.
Inspector Dave Sutton of Croydon police said: "Although the decision was taken to allow the event to continue while preventing further large numbers of people from gaining entry, police in Croydon will not tolerate violent disorder in any form.
"Anyone involved in the supply of illegal drugs, or who put themselves, police officers and members of the public in danger through acts of violence or criminal damage, should be aware that an extensive investigation has begun to identify, arrest and bring before the courts those who committed offences." Note: Police initially said six people had been taken to hospital but then revised that figure down to four. London Ambulance Service said this afternoon that seven people had been hospitalised.
