MANILA — Thousands of Filipinos poured out of their homes to join a funeral march on Saturday for Kian Loyd delos Santos, the 17-year-old boy whose death at the hands of the police has galvanized opposition to President Rodrigo Duterte’s brutal war on drugs.

Students joined nuns, activists and even supporters of Mr. Duterte as an estimated 5,000 people marched in light rain, demanding accountability from the president, who has appeared to soften his tough anti-crime rhetoric and has ordered the detention of three police officers pending an investigation into the killing.

“I hope that what happened to my son will not happen to members of their families,” Saldy delos Santos, the boy’s father, said of the police officers. He wore a white shirt with the words “Justice for Kian” written on an image of a black ribbon.