Thousands of Filipinos Saturday called for an end to extrajudicial killings as the funeral of a boy killed by police turned into the largest single demonstration yet against President Rodrigo Duterte's brutal drug war.

The killing of 17-year-old Kian Delos Santos last week triggered rare protests against Duterte's controversial but popular campaign to eradicate drugs, with critics saying it highlighted rampant rights abuses by police enforcing the crackdown.

Since Duterte's term began 14 months ago, police have reported killing 3,500 people in anti-drug operations, with thousands more murdered over drug-related crimes and in unexplained circumstances.

Duterte and his drug war are backed by a large majority of Filipinos fed up with high crime and a slow-moving judicial system, according to national polls.

But the killing of Delos Santos, the son of a poor sidewalk vendor and a migrant domestic worker, have dominated the media and sparked public outrage.

"We will pursue this fight. What happened to him was so unfair. We cannot let it stand," his 21-year-old cousin Jhai Delos Santos told AFP as she joined the protest march.

"We have rights too. They cannot just wage a drug war against people who have no drug records and are not taking drugs," she said, adding that the boy's father and grandfather have since received anonymous death threats.

Police said the teenager was a drug courier who fired at them while resisting arrest. However CCTV footage showed the two policemen dragging the unarmed boy away moments before he was killed.

Duterte, who had controversially drawn parallels between his drug campaign to Hitler's extermination of Jews and vowed to protect police from prosecution, has promised to bring the boy's killers to justice.

"The president has clearly stated that the war against drugs is not a license to break the law," Duterte's spokesman Ernesto Abella said in a statement issued late Friday.

- 'Stop Killing the Poor' -

After the boy's family held a wake for him at home, around 3,000 people including his classmates, neighbours, nuns, priests and human rights activists marched under cloudy skies to protest his killing, according to an AFP photographer at the scene.

"Kian is the name and face of the truth. We must not allow the truth to die with Kian's murder," said Father Robert Reyes, one of several Catholic priests who celebrated a church mass for the boy on Saturday.

Crowds lined the narrow streets as participants, many wearing black ribbons, carried posters and streamers that read "Stop Killing the Poor", "Justice for Kian", and "End Duterte's Fascist Drug War".

The cortege stopped briefly for prayers outside a police station where the three officers who had arrested the boy were deployed. They have since been suspended.

Following their claims of Delos Santos being involved in the drugs trade, police told a public enquiry on Thursday that they only read about his alleged narcotics activity on "social media" after his death.

A police autopsy also concluded the boy was fatally shot in the head twice as he lay prone on the ground.

Amnesty International alleged in a report released in February that Philippine police shot dead defenceless people, fabricated evidence, paid assassins to murder drug addicts, and stole from those they killed or the victims' relatives.

It also said police were being paid by their superiors to kill drug suspects, and documented victims as young as eight years old.

source: AFP