More than 150 people attended the unlicensed music event, which took place in a “hazardous” section of the building on Wythes Road in Royal Docks.
Police were called to the former pub shortly after 9.30am on Sunday following reports of anti-social and violent behaviour on the streets.
The Met said the event “recklessly breached” coronavirus regulations.
A 30-year-old man who organised the event could now be fined £10,000, police said.
Sergeant Simon Biggs said: “When I arrived on scene, I couldn’t believe what I was seeing and was shocked by the event organiser’s blatant disregard for the safety of those there.
“This health crisis has claimed thousands of lives and people attending events such as this risk furthering the spread of this deadly virus.
“The conditions at this abandoned pub were awful, there was no running water, poor sanitary conditions, no social distancing measures and most of those attending were not wearing a face covering.
“In addition to the obvious threat of the virus, the building was also evidently in a serious state of disrepair.
“Water was dripping from the ceiling and there were a number of other hazards. After leaving, officers made sure to secure the building to prevent further access.”
A large sound system was found inside the building and the crowd was dispersed by officers.
