The Metropolitan Police said it received a call at 21.14 GMT on Thursday, reporting a large gathering at a school in Egerton Road, Stamford Hill, north London.
The Met initially said officers found around 400 people inside the venue, but later revised the figure down to 150.
The group had gathered for a wedding party, the police found, adding that the building’s windows had been covered to stop people seeing inside.
The police are considering issuing a £10,000 ($13,641) fine to the organiser of the event, in accordance with government rules on COVID-19 breaches.
Five attendees of the wedding party were issued with £200 ($273) fixed penalty notices.
Detective Chief Superintendent Marcus Barnett of the Met Police said it was a “completely unacceptable breach of the law.”
“People across the country are making sacrifices by cancelling or postponing weddings and other celebrations and there is no excuse for this type of behaviour,” he said.
He said police officers “will not hesitate to take enforcement action if that is required to keep people safe.”
The strictly Orthodox Yesodey Hatorah Senior Girls’ School, whose venue was used for the wedding party, said it was “absolutely horrified” by what happened and “condemned it in the strongest possible terms.”
Under the new restrictions, people may only leave home for limited reasons, such as to shop for essentials, to work if they can’t work from home, to exercise, to seek medical assistance, or to escape domestic abuse.
