Philippine President Rodrigo Duterte's son has dismissed allegations of his involvement in the drug trade as "baseless", telling a Senate inquiry that he has no links to a massive narcotics shipment that has come under scrutiny.

Opponents of Mr Duterte, who has instigated a brutal crackdown on the country's drug trade, said they believed his son Paolo may have helped ease the entry of the $US125 million ($155 million) drug shipment from Beijing at the port in Manila.

"Once and for all, I now have the time to deny any and all baseless allegations thrown against me," Paolo Duterte told the Senate.

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

Senator Trillanes also said he had intelligence information from a foreign country that Paolo, the vice-mayor of Davao, was a member of a Chinese criminal syndicate, citing as proof a "dragon-like" tattoo with secret digits on his back.

When asked about the tattoo, Paolo admitted that he had one, but declined to describe it, invoking his right to privacy. When pressed if he would allow a photograph to be taken of the tattoo and sent to the US Drug Enforcement Agency to decode secret digits, Paolo refused.

Over the course of the hearing, Paolo also consistently refused to respond to questions about his finances and bank accounts, calling them "irrelevant".

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

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

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

Mr Duterte unleashed his bloody campaign the day he took office on June 30 last year, after promising Filipinos he would use deadly force to wipe out crime and drugs.

Police records show more than 3,800 people have died in police operations since July last year, and more than 2,100 other reported murders are linked to drugs.

Reuters/AP