Former Philippines president Rodrigo Duterte failed to show in person at the International Criminal Court on Friday at the opening of his crimes against humanity hearing over his deadly crackdown on narcotics.

The 79-year-old, the first ex-Asian head of state to face charges at the ICC, was following by video link during a short hearing to inform him of the crimes he is alleged to have committed, as well as his rights as a defendant.

Sounding frail and wearing a blue suit and tie, he spoke briefly to confirm his name and date of birth. The presiding judge allowed him to follow proceedings in absentia due to his long flight to The Hague.

Duterte stands accused of the crime against humanity of murder over his years-long campaign against drug users and dealers that rights groups said killed thousands.

In the prosecutor's application for his arrest, he said Duterte's alleged crimes were "part of a widespread and systematic attack directed against the civilian population in the Philippines."

"Potentially tens of thousands of killings were perpetrated," the prosecutor alleged of the campaign that targeted mostly poor men, often without proof they were linked to drugs.

Victims' families have welcomed the trial as a chance to get justice, while Duterte supporters believe he was "kidnapped" and sent to The Hague amid a spectacular fallout with the ruling Marcos family. (AFP)