Family, friends and dignitaries are attending the service at St. Mary's Cathedral.
Fagan died of a heart attack last week in London. Fagan served 31 years on the force, decorated for valor eight times. He became chief in 2003, a day after being cleared of obstruction charges in the Fajita Gate scandal involving his son. He retired in 2004.
Alex Fagan senior was 60 years old.