Their lawyer, Jude Josue Sabio, said both men’s statements were tantamount to accusations of “crimes against humanity” and they would take their case to the International Criminal Court in The Hague by April.

“Both Matobato and Lascanas are vital witnesses to prove that the Davao death squad was used by Duterte as a template, or as a strategy or policy, for crime control through extrajudicial killings since he became president,” Mr. Sabio said in an interview.

Their accusations came after a report by Human Rights Watch, which found that the police had falsified evidence to justify unlawful killings and that the majority of those killed were poor. The report urged the United Nations to create an “independent, international investigation” into the killings.

Rommel Banlaoi, the director at the Center for Intelligence and National Security Studies, said accusations against Mr. Duterte of human rights abuses should be handled domestically.

“There is still a general support on the war against drugs, but the opposition is raising the issue of extrajudicial killings as a pretext to challenge the reign of Duterte,” Mr. Banlaoi said. “If Duterte fails to perform and the Philippine economy deteriorates to a level that hurts the stomach of the middle class, not to mention the lower class, the opposition can muster support to topple his regime.”

Mr. Duterte has said the accusations against him were attempts to destabilize his administration.

“When will I stop? When you threaten me with extrajudicial accusation? That threat has been going on since I was a mayor,” he said on Tuesday. “Just try.”