A woman has been fined £10,000 for organising an unlicensed music event which was attended by about 150 people.
Hertfordshire Police were called to a property in Blanche Lane, South Mimms, at about 20:00 GMT on Wednesday.
One person was arrested on suspicion of assaulting a police officer and 12 people were fined £200 each.
The organiser, a 22-year-old woman from London, was issued with the fixed penalty notice for breaching coronavirus restrictions.
The force said it was continuing to investigate and it was "likely that further fixed penalty notices will be issued".
Officers searched a van at the property and found "a large quantity of alcohol". Music equipment was also seized.
Ch Supt Richard Liversidge said: "Thanks to vigilant local residents, who reported the large number of cars parked nearby, we were able to shut down the event before it had got into full swing - there were around 150 people in attendance when we got there, but we actually believe that the organisers were expecting at least 300 people to turn up."