Police who broke up an 18th birthday party after a noise complaint said a guest "did not believe" in coronavirus.
The Derby West response team said it did not receive "the warmest of welcomes" after being called to the party in the Abbey area of Derby on Monday night.
The force said officers "directed partygoers to leave the house", adding one person was fined.
Lockdown measures mean there are strict rules against any large gatherings.
"The government guidance is there to protect each and every one of us and the danger of this virus cannot be overstated," the force said in a statement.
"We would urge everyone to read the guidance and abide by both the wording - and the spirit in which it was written."
The birthday party is not the first gathering that has been dispersed by Derbyshire Police, with officers describing their "absolute shock" at finding dozens of people crammed into a house shortly after the government issued its warnings.
Police in the county have also had to break up a shisha party in the Peak District, while forces in other parts of the UK have also handed out fines to residents flouting the rules.
Chief Constable Peter Goodman told MPs on the Home Affairs Select Committee on Monday the force handed out 30 fixed penalty notices, half of which were handed out over the past weekend.