Former Philippine president Rodrigo Duterte was served an Interpol arrest warrant from the International Criminal Court (ICC) on his arrival at Manila's main airport on Tuesday, the government said.

The ICC has said it would pursue an investigation of suspected crimes against humanity related to Duterte's role in overseeing a bloody "war on drugs" that killed thousands of Filipinos.

Duterte had said on Monday in Hong Kong that he was ready to be arrested if the ICC issued a warrant and has repeatedly defended the anti-drugs crackdown. He has denied ordering police to kill drug suspects unless in self-defence.

The office of President Ferdinand Marcos Jr said it received an official copy of the warrant, which was served on Duterte by police. Duterte was now in custody, it said in a statement.

Duterte's former legal counsel Salvador Panelo said the arrest was unlawful, and said the police did not allow one of his lawyers to meet Duterte at the airport.

The firebrand Duterte unilaterally withdrew the Philippines from the ICC's founding treaty in 2019 when it started looking into allegations of systematic extrajudicial killings, and the Philippines had until last year refused to co-operate with the ICC investigation.

The "war on drugs" was the signature campaign policy that swept Duterte to power in 2016 as a maverick, crime-busting mayor, who delivered on promises he made during vitriolic speeches to kill thousands of narcotics dealers.

According to police, 6,200 suspects were killed during anti-drug operations that they say ended in shootouts. But activists say the real toll of the crackdown was far greater, with thousands of drug users in the country's slums, many of whom were included on official "watch lists," killed in mysterious circumstances.

Police deny involvement in those killings and reject allegations from rights groups of systematic executions and cover-ups.