Among those in attendance were Serbian Ambassador to Croatia Stanimir Vukicevic and Serbian minority MP Milorad Pupovac.
Patriarch Irinej's main message to those present was to keep their faith and affirm it with their lives.
Addressing the Orthodox faithful, he said they should show love to those with whom they shared their lives in Zagreb, their Catholic brothers.
The Patriarch said he was glad to be in Zagreb for the blessing of the cathedral and the Serbian Orthodox Secondary School Kantakuzina Katarina Brankovic that was to be inaugurated at 6 pm.
On Friday, Patriarch Irinej and the members of the Holy Synod will meet with the Archbishop of Zagreb, Cardinal Josip Bozanic, after which they will be received by Croatian President Ivo Josipovic and Prime Minister Zoran Milanovic.