Thousands of Roman Catholics marched in the Philippines capital Manila on Saturday in the biggest gathering denouncing extra-judicial killings and a government plan to reimpose the death penalty for criminals.

Dubbed a "Walk for Life" prayer rally and endorsed by the Catholic Bishops Conference of the Philippines (CBCP), the gathering came just days after the church launched its strongest attack against President Rodrigo Duterte's war on drugs.

Organisers claimed as many as 50,000 people took part in the march toward Manila's Rizal Park, while about 10,000 based on police estimates stayed to hear speeches.

More than 7,600 people have been killed since Duterte launched his anti-drugs campaign seven months ago. More than 2,500 died in shootouts during raids and sting operations, according to the police.

Amid mounting criticism about a surge in killings, Duterte said on Saturday that the campaign was "by and large successful".

Speaking at the Philippine Military Academy's alumni homecoming in Baguio City, he said the drug problem was more complex than he initially thought, prompting him to seek military support.

"I need the help of each one, especially the military, not for social control but protection (for) the citizens from the lawless, the reckless, and the selfish," the firebrand leader said.

Both the government and police have denied that extra-judicial killings have taken place. But human rights groups believe many deaths that police had attributed to vigilantes were carried out by assassins likely colluding with police.

"We cannot teach that killing is wrong by killing those who kill. It also increases the number of killers," CBCP president Lingayen-Dagupan Archbishop Socrates Villegas said in a statement.

Manila Archbishop Luis Antonio Cardinal Tagle, who also joined the rally, called for strengthening and promoting the culture of non-violent movements.

In its most strongly worded attack on the crackdown on drug pushers and users, a CBCP pastoral letter read out at services across the country early this month said killing people was not the answer to trafficking of illegal drugs.

Nearly 80 percent of the Philippines' 100 million people are Catholic and until recently the church had been hesitant to criticise Duterte's war on drugs.

Senator Leila de Lima, a staunch critic of Duterte's war on drugs now facing three drug-related charges, also joined the rally. She said the charges were meant to silence her.