Up to 100 people were cleared from a building by police amid reports of a party.
Officers were called to the building on Talbot Street in Nottingham just before 00:25 BST.
Nottinghamshire Police said an investigation was ongoing to establish if all the people cleared from the building were at the same party.
Hosts of two other parties, in Nottingham and Mansfield, have also been fined, the force said.
Officers issued a fixed penalty notice to a 40-year-old woman who held a birthday party for her boyfriend in Mansfield.
The woman tried to claim the party of 10 people was within government guidelines, the force said.
At another party, on Colville Street in Nottingham, the organiser claimed she did not have to follow the rules because she had already had Covid-19.
Police said she was fined after becoming "confrontational".
Deputy Chief Constable Rachel Barber said: "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. This puts themselves and others at risk.
"We all need to pull together and do our bit to protect everyone from this disease, which the vast majority are doing."
The whole of Nottingham and Nottinghamshire is currently under tier two restrictions, where no household mixing is allowed indoors.
Discussions to move the city and parts of the county into tier three are set to continue next week.