A member of the public called detectives to reports of a crowd at a home in Harrow at 11.30am on October 25.
Officers couldn’t get into the property due to the number of people inside for the religious service.
Attendees made no attempt to socially distance or wear a face covering, police said.
Police broke up the crowd and told the guests they needed to leave the property immediately.
It comes as police are preparing to escalate their response to lockdown breaches after arrests on the first weekend of the second national lockdown in England.
Under laws that came into force last Thursday, nobody is allowed to meet indoors or in a private garden with anyone from another household.
PC Sean Carroll, of the North West BCU, said: “The Coronavirus regulations have been put in place to protect us all and it is deeply disappointing that people continue to flout them.
“On this occasion, due to the seriousness of the breach, it was decided that the organisers should be reported for consideration of a fine. Large gatherings such as this can be a breeding ground for the virus, so we make no apologies for our determined efforts to shut them down and report the rule breakers.”
Officers discovered the unlicensed party in an outbuilding at Warren Farm Estate, Ealing, around 11.45pm on Saturday.
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