A woman who threw a house party told officers she was justified in organising it because she did not have Covid and "it was her birthday soon".
Hampshire Constabulary broke up the gathering in Durley Crescent, Totton, Hampshire, on Saturday evening.
About 20 adults from different households were present, with 10 women hiding in a garden shed when police arrived.
The 18-year-old party organiser will receive an £800 fixed penalty notice.
At about 10:50 GMT police received a report about a potential breach of Covid-19 regulations.
Temporary Sgt Robin Blake said: "This was a blatant breach of the Health Protection Regulations that are in force to keep people safe during the pandemic.
"Upon arrival we were met with obstructive behaviour.
"The 18-year-old woman tried to justify her actions by saying she doesn't have Covid and that it's her birthday soon.
"Despite pointing out the reasons why these restrictions are in place, her attitude was appalling.
"These people were putting themselves and their loved ones at risk. It is actions like this that continue to spread the virus and put unnecessary pressure on the NHS."
In January the government introduced fines of £800 for people attending house parties of more than 15 people.
They double for each repeat offence to a maximum of £6,400.
Officers took the details of all of those present at the property before dispersing them.