Former Philippines president Rodrigo Duterte believes he has no case to answer at the International Criminal Court, his daughter said Sunday, and is confident he has a strong legal argument.

Sara Duterte, vice-president of the Philippines, told reporters her father was now in better health after appearing fatigued and dazed at his first hearing at the ICC, reports AFP.

"He`s very confident about the legal arguments. He`s very confident... that what they did was wrong and there is no case to begin with," Duterte told reporters.

The 79-year-old former president faces a charge of crimes against humanity tied to his "war on drugs" in which thousands were killed.

He is currently in a detention centre in The Hague, awaiting a confirmation of charges hearing on September 23, where he will have the opportunity to contest the accusations against him.

Duterte was bundled into a plane in Manila and brought to the Hague via Dubai, appearing barely awake at his initial hearing two days later.

But his daughter, who said she saw him on Saturday, said: "He feels better now and thankfully the doctors and the nurses are taking good care of him."

She had previously revealed that her father`s main complaint was the Dutch food served in the centre.

"He`s getting rice. That`s what we asked for and it`s cooked perfectly. Correct according to Filipino taste," said Sara Duterte.

He was asking for his personal clothes and a supply of diet cola, she said.

Sara Duterte had earlier Sunday addressed a crowd of several thousand gathered in spring sunshine on the Malieveld in The Hague.

Supporters shouted "bring him home", with pro-Duterte demonstrators believing the former president was "kidnapped" to face the ICC.

His lawyer Salvador Medialdea told the court on March 14 his client had been "abducted from his country".

"He was summarily transported to The Hague. To lawyers it`s extrajudicial rendition. For less legal minds, it`s pure and simple kidnapping," said Medialdea.

In the ICC chief 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 ICC proceedings as a chance for justice, while Duterte supporters believe he was sent to The Hague amid a spectacular fall-out with the ruling Marcos family.