MANILA (Reuters, AFP) - Roberto Ongpin, one of the Philippines' richest men, resigned as chairman of a listed gaming company one day after President Rodrigo Duterte swore he would take down the country's "oligarchs" and singled out Ongpin as one of them.

Ongpin announced his resignation in a disclosure carried by the stock exchange, but did not give a reason for the move.

"I hereby tender my resignation with immediate effect as chairman and director of Philweb Corporation and all of its subsidiaries," read the statement.

Ongpin stepped down to focus on real estate projects developed by Alphaland Corp, also majority-owned by Ongpin, Philweb said.

Shares in gaming cafe operator Philweb Corp slumped as much as 50 per cent in Manila trading on Thursday. The stock closed 37 per cent lower.

Ongpin was once a trade minister under disgraced former president Ferdinand Marcos, whose regime was characterised by widespread corruption, decadence and military brutality.

Tycoons adds to a growing list of targets that Duterte is pursuing. The firebrand former prosecutor has said he will crack down on drug dealers, crime bosses, corrupt generals, business monopolies, online gambling and even Catholic priests.

"I am fighting a monster... Believe me, I will destroy their clutches in our nation," Duterte said at an environment summit in southern Davao City on Thursday, without identifying anyone.

"My order to the (assembly): Destroy the oligarchs that are embedded in government now. Those are the ones. I'll give you an example, publicly, in front of the nation: Ongpin, Roberto."

"These are the guys, who, while sitting inside their planes or their mansions everywhere, are raking in money like taxi metres."

Philweb won a government license to launch internet cafés exclusively dedicated to hosting casino games in 2003.

Duterte has previously vowed to fight corruption but his main focus has been a war on crime that has seen the killing of hundreds of suspected drug dealers since he was elected on May