I found out that there are 9 different species of bats.The best places to spot these flying dudes are in East Helsinki (e.g. Herttoniemi). Also good places are Seurasaari and Kivinokka (according to Helsinki environment council).
|Nyctalus Noctula (picture from here)|
Here you can see the ares in Helsinki, where you can spot bats (opens as PDF)