REVELLERS smashed up the former Royal Mail delivery office in East Croydon during an illegal rave.
Thousands of people descended on the venue from around 9pm yesterday (Saturday) evening.
Missiles, including fire extinguishers and furniture, were reportedly thrown at police when they initially tried to shut the event down.
Nine people were arrested and six others were taken to hospital.
Croydon Central MP Gavin Barwell criticised those responsible for the rave and the damage, saying they should "feel the full force of the law".
Council leader Tony Newman said: "Thankfully unlicensed raves are very rare in our borough and we will be working with our partners to make sure such a situation does not happen here again."
At around 11pm the police, who had called in extra resources from across London, decided to let the rave continue to avoid further trouble.
The event went on into the early hours and saw some partygoers cause extensive damage to the building.
Officers eventually moved in and have now cordoned off the site and have led people out of the depot.
The scene from outside shows dozens of smashed windows, glass over the pavement and even fire extinguishers littering the floor.
Trams are not currently running through East Croydon station, which is opposite the building.
A security firm is on site attempting to secure the depot.
Writing on his blog, Mr Barwell said that police found around 200 people inside, and further 300 outside, when first called to the building at 9pm.
He added: "There appeared to be only one entry and exit point to the venue raising serious concerns about the health and safety of those inside if there was a fire or other emergency.
"As police sought to secure the building in an attempt to prevent further attendance, the crowd outside rapidly swelled to excess of 1,000.
"At this point additional police resources were requested from across London.
"Within 30 minutes the crowd had doubled and was increasing further with every train that arrived at East Croydon station.
"Officers became the the focal point of the crowd's frustration and missiles began to be thrown at the police.
"This then escalated to fire extinguishers and furniture being thrown from the building at the officers below."
At 11pm the decision was taken "turn the building over to the crowd", said Mr Barwell.
"It was evident that the officers on scene were at risk of serious injury and there was a distinct risk of the crowd moving into a very busy Croydon town centre with the possibility of serious public disorder.
"Instructions were given for all officers to withdraw from the building, but to maintain a watching brief to coordinate with London Fire Brigade and ambulance service should the need to enter the building arise due to serious injury or fire.
"Individuals continued to sporadically smash windows and throw items down onto the pavement, including masonry and more fire extinguishers.
Cllr Newman said: "Thanks to the efforts of the police there were no serious injuries to contend with.
"We have begun a full clear-up operation of the area and surveyors have been brought in to help make the building safe and secure. We will also be discussing responsibility for security of the site with the Royal Mail."
The delivery depot closed earlier this year to make way for flats and was relocated to Factory Lane.
It is understood the building had been occupied by squatters this week.
In April, an illegal rave in a warehouse in Bath House Road, just across the border in Sutton, attracted a crowd of more than 3,000 people.
