MANILA (AFP) - Church bells were set to toll across the mainly Catholic Philippines on late Thursday (Sept 14) as bishops rallied opposition to the "reign of terror" that has left thousands dead in President Rodrigo Duterte's drug war.

Police have reported killing more than 3,800 people to fulfil Duterte's vow to rid the country of narcotics, with the 15-month crackdown triggering wider violence that has seen thousands of other people found dead in unexplained circumstances.

The Church said bells around the country would simultaneously ring for five minutes at 8pm (1200 GMT) to honour the dead and remind the living that the bloodshed must stop. The ritual will continue for 40 nights.

"We cannot allow the destruction of lives to become normal. We cannot govern the nation by killing," Cardinal Luis Antonio Tagle said in a pastoral letter last week launching the campaign.

The president of the Catholic Bishops Conference of the Philippines, Archbishop Socrates Villegas, followed up this week with an even stronger pastoral letter.

"For the sake of the children and the poor, stop their systematic murders and spreading reign of terror," Archbishop Villegas wrote.

Mr Duterte won last year's presidential elections on a brutal law-and-order platform in which he promised an unprecedented campaign to eradicate illegal drugs in society by killing up to 100,000 traffickers and addicts.

Mr Duterte has made the drug war the top priority of his administration, and has regularly encouraged more bloodshed with comments such as describing himself as "happy to slaughter" three million addicts.

Nevertheless, the president and his aides reject allegations they are overseeing a crime against humanity.

They say police are killing only in self-defence, and the thousands of other unexplained murders could be due to drug gangs fighting each other.

Many Filipinos looking for quick solutions to crime continue to support Mr Duterte, according to polls, and he enjoys majority backing in both houses of Congress.

But the Church has emerged as the leader of a growing opposition in recent months.

The killings of three teenagers, two of them at the hands of Manila police, sparked rare street protests against the crackdown.

Church officials say the tolling of bells is a direct throwback to the Crusades in the Medieval Age, when Christian nations of Europe sent military expeditions to reclaim holy places in the Middle East.

The Catholic Church, to which eight in 10 Filipinos belong, has a history of influencing politics in the Philippines and helped lead the "People Power" revolution that overthrew dictator Ferdinand Marcos in 1986.

Mr Duterte has repeatedly praised Mr Marcos as a "hero", and made speeches seeking to discredit the Church.