The Roman Catholic Church in the Philippines slammed President Rodrigo Duterte’s war on drugs in a sermon delivered during Sunday’s services, marking its strongest opposition yet to a flagship policy that has led to the deaths of more than 7,000 people in the last seven months.

In a pastoral letter by the Catholic Bishops’ Conference of the Philippines, church leaders called Mr. Duterte’s antinarcotics campaign a “reign of terror in many places of the poor.” They said that while illegal drug trafficking needs to be stopped,...