Duterte came to power last year promising to wage a brutal and unprecedented war on drugs. Since then, police have killed around 3,800 suspects in anti-narcotics operations, while thousands of other people have been murdered in unexplained circumstances.
His son Paolo Duterte and son-in-law Manases Carpio appeared at a Senate inquiry Thursday to answer accusations that they helped facilitate the shipment of crystal methamphetamine worth 6.4 billion pesos ($125.4 million) into the Philippines from China in exchange for payment.
The allegations emerged last month when a customs broker told the Senate panel that he had heard the names of Duterte's son and son-in-law mentioned while seeking to expedite the shipment.
The broker later issued a statement clearing the two men of involvement.
But opposition senator Antonio Trillanes at the inquiry accused Duterte's son of belonging to a drug trafficking gang, saying the 42-year-old had a tattoo on his back, resembling a dragon, that was "proof... of his membership" in the gang.
Paolo Duterte, vice-mayor of the southern city of Davao, told the panel that he had a tattoo but said he could not answer allegations based on hearsay.
"The law of karma will operate especially to those with evil intent," he added in an apparent swipe at Trillanes, an arch-critic of the president.
Carpio, who is married to Davao city mayor Sara Duterte, also rejected allegations against their family.
"I and my brother-in-law have been publicly crucified based on rumours and gossip," said Carpio, a lawyer.
"I have no knowledge of or involvement in the illegal drugs shipment."
The sensational allegations have captured the attention of Filipinos, many of whom back the president's vow to pursue an unrelenting war against drug traffickers.
Duterte, 72, has said he would be "happy to slaughter" three million Filipino drug addicts, even as critics warn the deaths of thousands of people killed in the crackdown may amount to a crime against humanity.
Duterte has denied ever inciting police or vigilantes to commit mass murder.
In speeches, he had encouraged his son to attend the hearing but advised him to invoke his "right of silence".
Detained opposition senator Leila De Lima last month accused the president of hypocrisy over his son's case.
"Let us look at the issue involving the 6.4 billion pesos worth of (illegal drugs) in the Bureau of Customs where the presidential son Paolo Duterte is being implicated," she said.
"(Rodrigo) Duterte has said a mouthful about killing the poor caught with small grams of shabu but his silence on the tonnes of illegal drugs that slipped past customs is deafening."//AFP