The Philippine Senate said on Monday it will conduct a formal probe of ex-president Rodrigo Duterte's arrest and swift handover last week to the International Criminal Court, which is to try him for alleged crimes against humanity.

The 79-year-old, the first Asian former head of state charged by the ICC, stands accused of the crime against humanity of murder over his years-long campaign against drug users and dealers that rights groups have said killed thousands.

The probe was initiated by Senator Imee Marcos, sister of President Ferdinand Marcos but a close friend of Duterte's eldest daughter, Vice President Sara Duterte.

The two families have had a spectacular falling out since Marcos teamed with Duterte to win an election landslide in 2022. The latter has since been impeached on charges that include an alleged assassination plot against the president.

"As chairperson of the Senate Committee on Foreign Relations, I am calling for an urgent investigation into the arrest of former President Rodrigo Roa Duterte, an issue that has deeply divided the nation," Imee Marcos said on Monday.

"It is imperative to establish whether due process was followed and to ensure that his legal rights were not just upheld but protected," she said, adding: "Our sovereignty and legal processes must remain paramount."

Duterte was arrested at Manila airport on March 11 after a brief trip to Hong Kong and flown to the Netherlands just hours later where he was turned over to the ICC.

The Senate has set a public hearing for Thursday and invited top police and other government officials to give evidence.

Imee Marcos has tracked a course largely independent from her brother on many issues, though she is running for re-election under the administration ticket for the May 12 midterm elections.

Hours after the arrest, she challenged the wisdom of the arrest of "poor president Duterte", warning that "this can only lead to trouble".

On Friday, she declared: "I cannot accept what they did to [Duterte]." (AFP)