A child's first birthday party in Mansfield was broken up after police found 24 people celebrating in a small flat.
A number of the people inside had even travelled from Birmingham, officers said.
Fourteen adults were fined a total of £11,200, with the coronavirus restrictions banning different households from mixing inside.
New laws recently increased fines to £800 per adult for gatherings of above 15 people inside a dwelling.
Members of the public reported that they had seen party banners at the family barbecue and gathering in Mansfield.
Officers attended the flat in Victoria Street at around 3pm on Thursday, February 4, to find 14 adults celebrating the birthday with 10 children.
Assistant Chief Constable Steve Cooper, of Nottinghamshire Police, said: "It's very disappointing to hear that officers had to break up another large gathering in the county.
"It is clear that there is still a minority of people who think the rules do not apply to them or that they can make up their own versions of the guidelines.
"When we attend a party we will firstly encourage people to disperse safely.
"We would appeal to all local residents to comply with the Covid-19 regulations which are there to protect everyone.
"It is the case that people are being issued fines for breaching the regulations, but more importantly the virus is spreading rapidly throughout the community and people are dying."
He added: "It's important to highlight that officers are putting their own health, as well as their families’ health, on the line by attending these kind of house parties night after night.
"We would like to continue to praise their bravery on attending these mass gatherings and working so hard in difficult and anxiety-filled circumstances.
"Due to the nature of the offences, it was deemed appropriate to issue all the adults inside the address fixed penalty notices and then send them home."
