Patriarch Maxim died last Tuesday of heart failure. He was 98.
The Bulgarian Church's Holy Synod of 13 senior clergy will meet to choose an interim patriarch until a larger Church Council is held within the next four months to pick Maxim's successor, church officials said.
Orthodox Christianity is Bulgaria's dominant religion, followed by more than 80 percent of the country's 7.4 million people.
Maxim was the church's leader for more than four decades, bridging the country's transition from communism.
See the full text of Pope Benedict's telegram on the death of Patriarch Maxim below:
To His Eminence Grigorij di Veliko Trnovo
Interim President of the Holy Synod of the Orthodox Church of Bulgaria
It is with deep sorrow that I heard the news of the death of our beloved brother in Christ, His Holiness Maxim, Metropolitan of Sofia and Patriarch of Bulgaria, who served our Lord and his people with devotion for many years.
On behalf of the Catholic Church, I wish to assure you and all the bishops, priests, and faithful of the Orthodox Church of Bulgaria that I join in prayer in your grief. May the Lord, Who is good and merciful, welcome our beloved brother Maxim into His heavenly home. May He grant him peace and eternal memory!
Participating in the pain of the Bulgarian Orthodox Church, I thank God for all the blessings of the late Patriarch for his church and the people of his country.
In particular, I remember the warm hospitality accorded to Blessed Pope John Paul II during his visit to Bulgaria in May 2002.
I thank the Lord for the good relations that the Patriarch has fostered with the Catholic Church in these lands, and I sincerely hope that the good relations can continue in order to promote the proclamation of the Gospel.
With renewed expressions of sympathy and the assurance of my remembrance and of my prayer, and I ask you to accept, your Eminence, the expression of my sincere greetings in Christ.