Thousands of Catholics have gathered in the Philippine capital in a "show of force" to protest extrajudicial killings being carried out under the banner of President Rodrigo Duterte's war on drugs.

The rally, dubbed the "Walk for Life", was attended by 20,000 people, organisers said. Manila police estimated the crowd at 10,000.

At the biggest rally yet against the killings, members of one of the nation's oldest and most powerful institutions prayed and sang hymns as they marched before dawn on Saturday, to condemn a "spreading culture of violence".

More than 7,000 people have died since Duterte took office almost eight months ago and ordered an unprecedented crime war that has drawn global criticism for alleged human rights abuses.

The move, however, has been popular with many in the mainly Catholic nation.

"We have to stand up. Somehow this is already a show of force by the faithful that they don't like these extrajudicial killings," Manila bishop Broderick Pabillo told AFP news agency before addressing the crowd.

"I am alarmed and angry at what's happening because this is something that is regressive. It does not show our humanity."

The demonstrators also condemned legislation restoring the death penalty for drug-related crimes and other offences.

Duterte, 71, has attacked the Church as being "full of sh*t" and "the most hypocritical institution" for speaking out against a campaign that he says would save generations of Filipinos from the drug menace.

About 80 percent of the 100 million Filipinos are Catholic.

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The Church helped lead the revolution that toppled dictator Ferdinand Marcos in 1986 and a 2001 uprising against then-president Joseph Estrada that saw him ousted over corruption charges.

The Church had initially declined to voice opposition publicly to Duterte's drug war but, as the death toll of mostly-poor mounted, it started late last year to call for the killings to end.

"It is obvious that there is a spreading culture of violence. It is saddening to see, sometimes it drives me to tears, how violent words seem so natural and ordinary," said Manila Cardinal Luis Tagle, the country's highest-ranking Church official.

Dawn, which is supposed to be the hour of a new start, is becoming an hour of tears and fears Archbishop Socrates Villegas, president of the bishops' conference

"If the response to violence is also violence, then we are only doubling down on violence."

The Catholic Bishops' Conference of the Philippines had called on the faithful to gather before dawn at the Quirino Grandstand, in the same venue where Duterte held a huge pre-election rally in 2016.

"Why dawn? It's because it is during these hours that we find bodies on the streets or near trash cans. Dawn, which is supposed to be the hour of a new start, is becoming an hour of tears and fears," Archbishop Socrates Villegas, president of the bishops' conference, told the crowd.

Among those who attended Saturday's event was Senator Leila de Lima, a former human rights commissioner, who is one of Duterte's most vocal opponents.

The government on Friday filed charges against her for allegedly running a drug trafficking ring inside the country's largest prison when she was justice secretary in the previous administration.

De Lima, who has denied the charges, said she attended the event as a show of solidarity.