Finland's health institute warned football fans returning home from Russia to get a COVID-19 test and self-isolate.
In a statement released on Saturday, the institute said the number of cases detected among fans who travelled to Saint Petersburg had increased.
They urged fans who travelled by bus to get tested immediately.
"Infections have been detected in passengers to such an extent that any bus or minibus may have been exposed during the journey," the Finnish institute for health and welfare said.
Finland played two matches in Saint Petersburg during the group stage of the 2020 UEFA European Football Championship.
A total of six matches took place in Saint Petersburg and one more match is set to take place there during the quarterfinals.
Russia has recently reported a significant rise in daily COVID-19 cases and the city of Saint Petersburg had a record daily death toll on Saturday.
Finland's health institute said those returning from St. Petersburg by bus should "avoid contact with people outside the family and apply for a coronavirus test 72 hours after arrival".
Some of the infections were traced back to the Alpenhaus restaurant in the city, the health institute added.