The venue was a school whose principal died from the coronavirus last year.
The Metropolitan Police force said officers found hundreds of people packed into the north London school with blacked-out windows on Thursday night. The force said that "following enquiries it was established that the group had gathered at the location for a wedding."
Weddings are allowed only in "exceptional circumstances" -- such as one partner being dangerously ill -- and with a maximum of six people attending.
Police said the organizer could face a 10,000-pound ($13,600) fine, Many guests fled as police arrived, but five people who attended received 200-pound fines.