More than 300 people were exposed to the coronavirus in South Savo's town of Mikkeli, according to local health officials.
The South Savo Social and Health Care Authority (Essote) said contact tracers have not been able to reach everyone thought to have been exposed to the virus. It added that the number of people who may have been exposed to infection may increase significantly.
A total of 21 people in the Essote region have tested positive for Covid-19. The health authority said that many who had the virus had been moving about in the city at times when they still were contagious.
In practice, this meant that the town's entire downtown area --including hundreds of business establishments--was effectively a zone in which people could have been exposed to coronavirus during the weekend of 5-6 September.
Essote's director of health Santeri Seppälä, said the majority of the new infections were seen in people who had been in the town centre over the weekend.
He advised that people who were in the area on Saturday or Sunday should avoid contact with other people for at least a week and should monitor their health for signs of coronavirus symptoms.
Mikkeli is a town with a population of nearly 54,000, located about 240 km northeast of Helsinki.
Meanwhile, Finland recorded 93 new lab-confirmed Covid-19 cases on Wednesday, according to the Finnish Institute for Health and Welfare, THL. The total number of cases in Finland now stands at 8,430.