Philippine President Rodrigo Duterte's son has told a Senate inquiry he had no links to a seized shipment of $125m worth of narcotics from China, dismissing as "baseless" allegations of his involvement in the drug trade.

Opponents of Rodrigo Duterte, who has conducted a crackdown on a trade he says is destroying the country, say they believe his son Paolo may have helped ease the entry of the drug shipment at the port of Manila, the capital.

"I cannot answer allegations based on hearsay," Paolo Duterte, vice mayor of the southern city of Davao, told the Senate on Thursday.

"My presence here is for the Filipino people and for my fellow Davaoenos whom I serve."

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On Tuesday Duterte said he had told Paolo to attend the Senate investigation if he had nothing to hide, besides advising him not to answer questions and invoke his right to keep silent.

He was referring to the people of Davao, where his father served as mayor for more than two decades before being elected president in 2016.

Duterte has repeatedly said he would resign if critics could prove any members of his family were involved in corruption.

Senator Antonio Trillanes, a staunch critic of Duterte, showed the Senate panel photographs of Paolo Duterte beside a businessman who was behind the shipment in which the alleged drugs were found.

Previous involvement

President Duterte's son-in-law, Manases Carpio, who has also been accused of links to the May drug shipment from China, told the hearing he had no involvement.

Al Jazeera's Jamila Alindogan, reporting from Manila on Thursday, said this was not the first time Duterte's son has been implicated in drug cases.

"There is government documentation as far back as 2007 showing that Paolo Duterte had been implicated by the drug enforcement agency as one of the drug protectors in Davao city, in their hometown in the southern Philippines," she said.

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She said many rights groups who oppose Duterte's bloody war on drugs are calling out Paolo Duterte and Carpio for their privilege.

"Here at the Senate hearing, the president's family members are given the chance to explain their side, to read their opening statements, have the best lawyers and basically have due process in court," our correspondent said, pointing out that those who were killed by police officers in impoverished communities were not afforded the same.

"It remains to be seen how this will end and how this will affect the popularity of the president," she said.

Trillanes said he had intelligence information from an undisclosed foreign country that Paolo Duterte was a member of a criminal syndicate, citing as proof a "dragon-like" tattoo with secret digits on his back.

Asked about the tattoo, Duterte said he had one, but declined to describe it, invoking his right to privacy.

Asked by Trillanes if he would allow a photograph to be taken of the tattoo and sent to the US Drug Enforcement Agency to decode the secret digits, Duterte said: "No way".

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He refused to respond to questions about his bank accounts, calling them "irrelevant".

Ernesto Abella, presidential spokesperson, said the attendance of Duterte and Carpio "demonstrates that both gentlemen are willing and ready to face malicious allegations intended to impugn their character and credibility".

Duterte won last year's elections after promising an unprecedented crackdown to eradicate drugs, which eventually led to tens of thousands of deaths.

Since then police have reported killing 3,800 people in anti-drug operations, with thousands of other people murdered in unexplained circumstances.

'Happy to slaughter'

Police reject activists' allegations that they are executing suspected drug users and dealers and say officers shoot only in self-defence.

Duterte has said he would be "happy to slaughter" three million Filipino drug addicts, and that he would pardon police if they were found guilty of murder for killing as part of his drug war.

But he has denied ever inciting police or vigilantes to commit mass murder.

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Source: Al Jazeera and news agencies