Forssell found the net three times in the second half and Jari Litmanen scored a penalty on his farewell international fixture.
A second half explosion handed interim coach Olli Hattanen, who took over from Stuart Baxter after the English-born tactician’s contract with the national team expired last week, a win in his debut for the Finns.The hosts scored just once in the first-half through Mika Vayrynen but the floodgates opened up after the interval with seven goals, including a hat-trick for Mikael Forssell.
The former Birmingham fans' favourite finished his third in style into the top corner with San Marino goalkeeper Aldo Simoncini rooted to the spot.
Kasper Hamalainen scored twice, Roni Porokara added his name to the scoresheet as well and the goal that all Finnish fans wanted to see, Jari Litmanen converting a penalty on his last appearance for the national team.
It was a convincing victory and Finland are now off the mark in Group E with three points while San Marino are still looking for their first points in the section.