Organizers had made preparations for 8,000 to 10,000 people to take part in the service which was officiated by Bishop Eero Huovinen. Communion was distributed to the crowd by 80 Lutheran ministers and 80 deacons and other church workers.
No services were held in any of the diocese's 39 other churches on Thursday. Congregations had been urged to attend at the Senate Square.
The square was also the site Thursday afternoon of a concert marking the 150th anniversary of the Finnish Evangelical Lutheran Mission.