The Philippines' Catholic Church will slam President Rodrigo Duterte's war on drugs at sermons across the country for creating a "reign of terror" among the poor.

Duterte has largely been supported in his crackdown on drug pushers and users, but the influential Catholic Bishops' Conference of the Philippines will issue a strong rebuke of his actions to date this weekend.

More than 7,600 people have been killed since Duterte launched his crusade against drugs seven months ago, with campaigners accusing him of crimes against humanity.

Letters outlining the church's concerns have been obtained by Reuters and will be read in services on Saturday evening (4 February).

"[A] cause of concern is the reign of terror in many places of the poor. Many are killed not because of drugs. Those who kill them are not brought to account," the letter said.

The Catholic Church in Philippines has previously voiced concerns of Duterte's extra-judicial killings, but never so strongly for fear of reprisal.

Duterte has routinely attacked the Church and has called for a "showdown" with priests whom he has accused of having wives, engaging in homosexual acts, misusing state funds and molesting children.

The church's rebuke will not mention Duterte by name but will urge "elected politicians to serve the common good of the people and not their own interests".

It also makes reference to "rogue policemen" who have routinely carried out the extra-judicial killings and the "corrupt judges" that let the killing take place unquestioned.

Philippine President Rodrigo Duterte
Philippine President Rodrigo Duterte's 'war on drugs' has led to thousands of deaths. REUTERS/Ezra Acayan

"Every person has a right to be presumed innocent until proven guilty," the letter added.

Nearly 80% of the Philippines' 100 million people are Catholic and, unlike in many other countries where the faith has waned, the majority still practice with enthusiasm.