As many as 300 people are thought to have participated in the party in Hackney on Saturday night.
Metropolitan Police officers had received intelligence that an unlicensed music event might be taking place ahead of the event.
Hundreds of people were later discovered in a small venue under a railway arch in Nursery Road.
Police said they made numerous attempts to try to speak with the organisers, but they were uncooperative.
At one point, officers said doors were padlocked from the inside the venue to prevent police from gaining entry.
After deploying additional units, including riot police, a dog unit and a helicopter, officers were able to force entry to the building.
Dozens of partygoers sought to escape by climbing fencing at the rear of the arch, while others forced open a gate to get away.
Police issued 78 people with fixed penalty notices of £200 each for breaching Covid regulations by attending an illegal gathering.
Chief Superintendent Roy Smith, who attended the incident, condemned partygoers’ actions.
“This was a serious and blatant breach of the public health regulations and the law in relation to unlicensed music events,” he said.
“Officers were forced, yet again, to put their own health at risk to deal with a large group of incredibly selfish people who were tightly packed together in a confined space – providing an ideal opportunity for this deadly virus to spread.
“Our frontline officers continue to police this public health crisis with compassion and professionalism but it is completely unacceptable they have to face such needless risks to their own health and to their families too.
“Whilst we will always seek to engage and explain, the message to those who have a total disregard for the safety of others and breach the law so blatantly is clear, our officers will act swiftly and decisively. Not just organisers, but all those present at such illegal parties can expect to be issued a fine.”