MANILA: The Philippines arrested firebrand former President Rodrigo Duterte on Tuesday at the request of the International Criminal Court, a major step in the world body's investigation into thousands of killings in a bloody "war on drugs" that defined his presidency.

Duterte, the maverick former mayor who led the Philippines from 2016 to 2022, was served an arrest warrant on arrival from Hong Kong at Manila's main airport and was now in custody, the office of his successor Ferdinand Marcos Jr said in a statement.

The "war on drugs" was Duterte's signature campaign platform that swept the mercurial, crime-busting former prosecutor to power in 2016 and he soon delivered on promises made during vitriolic speeches to kill thousands of drug dealers and users.

If transferred to the Hague, he could become Asia's first former head of state to go on trial at the ICC.

Duterte has insisted he told police to kill only in self-defence and has repeatedly defended the crackdown, saying he was willing to "rot in jail" if it meant ridding the Philippines of drugs.

In a video posted on Instagram by daughter Veronica Duterte from Manila's Villamor Air Base, where he has placed in custody, the former leader questioned the reason for his arrest.

"What is the law and what is the crime that I committed?" he said in the video. It was unclear who he was speaking to.

"I was brought here not of my own volition, it is somebody else's. You have to answer now for the deprivation of liberty."

SLUMLAND KILLINGS

The president's office has yet to clarify the next steps for Duterte and it was not immediately clear what the ICC has charged him with.

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 Duterte's crackdown was far greater, with many thousands more slumland drug users, some named on community "watch lists", killed in mysterious circumstances.

The ICC's prosecutor has said as many as 30,000 people may have been killed by police or unidentified individuals.

Police have rejected allegations from rights groups of systematic executions and cover-ups.

Duterte's arrest follows years of him rebuking and taunting the ICC since he unilaterally withdrew the Philippines from the court's founding treaty in 2019 as it started looking into allegations of systematic murders of drug dealers on his watch.

The ICC is probing alleged crimes against humanity and says it has jurisdiction to investigate alleged crimes that took place while a country was a member. The Philippines had refused to cooperate but the Marcos administration changed tack in November and started signalling it would comply if an arrest warrant was issued.

That came just hours after a remarks by Duterte in a legislative enquiry when he urged the ICC to "hurry up" with its investigation.

"I am already old, I might die soon. You might miss the pleasure of seeing me standing before the court hearing the judgement whatever it is," Duterte said, adding he assumed full responsibility for what happened.

ARREST 'UNLAWFUL'

News outlets earlier on Tuesday showed video footage of Duterte dressed in a jacket and striped polo shirt and walking casually through a corridor at the airport upon his return from Hong Kong with several police officers behind him.

Duterte's ally and former legal counsel Salvador Panelo said the arrest was unlawful and police had denied the former president legal representation.

"The ICC arrest warrant comes from a spurious source, the ICC, which has no jurisdiction over the Philippines," Panelo said in a statement.

Human rights groups said the arrest was a key step towards accountability for the killings of thousands of people in the Philippines.

Randy delos Santos, the uncle of a high school student Kian delos Santos, whose killing by police captured national attention, called the arrest "true justice".

"At least he is given the chance to defend his side, unlike the victims of his war on drugs," he said of Duterte.

Leila de Lima, a former senator who was jailed under Duterte months after she led an investigation into the drugs killings, said the victims' families had fought courageously for justice.

"Duterte is being made to answer - not to me, but to the victims, to their families, to a world that refuses to forget," she said.