Police broke up a party at a commercial unit on a Cardiff industrial estate.
Officers said they were called to a "commercial premises" at Wentloog Buildings, Rumney, in the early hours of Sunday.
South Wales Police said they found a a social gathering of 40-plus people "including a DJ set".
Lockdown laws in Wales stipulate that only people from two separate households are allowed to meet up outside.
The premises was cleared and closed and spokesman said that the owner of the premises was fined for breaching Covid-19 legislation.
The latest Welsh Government guidance on the coronavirus regulations says: “Coronavirus spreads much more effectively indoors because it can survive in the air and on surfaces for a very long time without direct sunshine. In addition to requiring people to stay local, therefore, the law also imposes restrictions on people being indoors with others without a reasonable excuse.
“The basic legal position is that you should not enter someone else’s home, allow someone from outside your household into yours, or meet with someone from another household indoors.
“The law includes reasons why it is permitted in limited circumstances to go indoors.”
