Several Finnish fans have tested positive for the coronavirus upon their return from Russia following a Euro 2020 group stage game against Belgium on Monday in St Petersburg.
Finnish media reported on Friday that 86 people had tested positive for the coronavirus so far.
Hundreds of Finns had travelled to Russia to follow the match.
Denmark meanwhile reported 16 coronavirus positive tests from fans who watched their final group game against Russia in Copenhagen, including four with the more infectious Delta variant.
All fans who watched that match from behind the goal in Stand B are being asked to take PCR tests.
Nine fans tested positive for the Delta variant following the previous match with Belgium.
Not all tests taken at the Finnish border have been analysed yet and a further increase in cases is expected.
About 800 people entered the country without being tested because the testing capacities at the border were not sufficient.
The Finnish health authority THL advised fans returning from Russia to get tested no more than 72 hours after the trip and to avoid contacts in the meantime.
The pandemic situation in Russia got worse rapidly in June.
New infection numbers rose across the country, including in major cities such as Moscow and Euro 2020 host city St Petersburg.
Lothar Wieler, president of German disease control body, Robert Koch Institute, said on Friday it does not make sense for people to travel all over Europe amid the coronavirus pandemic.
Wieler also criticized the staging of knock-out games in London and showed little sympathy for full stadiums.
“From the point of view of infectious medicine, this is not a good idea,” he said.
Finnish Prime Minister Sanna Marin expressed concern about the situation and also about people returning from a European Union summit in Brussels.
Finland has long had one of the lowest infection incidences in Europe.
Without specifying Euro 2020, the World Health Organisation’s lead coronavirus expert Maria Van Kerkhove said big events were a big risk.
“The events that are happening will have consequences. We are already starting to see some consequences of these events with increasing transmission again,” she told reporters.
“There is a lot that all of us want to be doing, but there is not a lot that we need to be doing right now.”