More than a thousand mourners attended a funeral procession in a northern Manila suburb on Saturday for a high school student whose killing a week earlier by anti-drug officers has caused rare public outrage about the country's war on drugs.

Kian Loyd delos Santos was dragged by plainclothes policemen to a dark, trash-filled alley in northern Manila, shot in the head and left next to a pigsty, according to witnesses whose accounts appeared to be backed up by CCTV footage.

The death of the 17-year-old has drawn huge domestic attention to allegations by activists that police have been systematically executing suspected users and dealers, a charge the authorities deny.