Police said the owner of the club in the northern port city of Gdańsk claimed the venue was hosting a practice session in which contestants were training for a dance competition next year.
Prosecutors will investigate whether the event on Saturday posed a public health threat that put lives at risk. A court could impose an eight-year jail sentence for such an offence, public broadcaster Polish Radio's IAR news agency reported.
Sanitary authorities will also investigate the incident, and could punish the club owner with a fine of PLN 30,000 (some EUR 6,700, USD 8,150), the IAR agency added.
Gatherings of more than five people have been forbidden in Poland under rules aiming to contain the COVID-19 epidemic, which has killed over 27,000 people across the country.
Poland on Monday entered a three-week “national quarantine” as it battles the coronavirus.