Police busted a wedding attended by around 400 guests in north London.
The organiser now faces a £10,000 fine for putting on the illegal event, which took place inside a school on Egerton Road in Stamford Hill.
The windows of the building had been covered to stop people seeing in when officers arrived on Thursday night.
The Metropolitan Police said the event was attended by around 400 guests - but many fled when officers arrived at the scene.
Five people were handed fines of £200 for attending the gathering, while the organiser will be reported for consideration of a £10,000 fine, police said.
Detective Chief Superintendent Marcus Barnett said: “This was a completely unacceptable breach of the law, which is very clearly in place to save lives and protect the NHS.
“An NHS that is under considerable pressure at a time when Covid-19 has killed nearly 100,000 people.
“This is a deadly and very dangerous disease. We can all see that and we must act responsibly.
“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.
“My officers are working tirelessly with the community and we will not hesitate to take enforcement action if that is required to keep people safe.”
The current national lockdown guidance states that weddings may only take place "in exceptional circumstances".
"This may be for example, if you or your partner is seriously ill and not expected to recover, or is to undergo debilitating treatment or life-changing surgery," the guidance says.
Ceremonies that do go ahead must only take place with up to six people.
