Davao City Vice Mayor Paolo Duterte, son of Philippine President Rodrigo Duterte, takes an oath as he attends a senate hearing in Manila on September 7, 2017.

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

Opponents of the president, who has instigated a fierce 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 in Manila, the capital.

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.

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

"My presence here is for the Filipino people and for my fellow Davaoeños whom I serve," he added, referring to the people of Davao, where his father served as mayor for more than two decades before being elected president in 2016.

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