Duterte won May's election in a landslide after a campaign dominated by his pledge to end crime within six months, promising to do so by unleashing security forces with shoot-to-kill orders.
In his inauguration speech on Thursday, Duterte insisted his adherence to the rule of law was "uncompromising", apparently seeking to assuage concerns from human rights groups that he planned to orchestrate mass extrajudicial killings.
But today, Duterte called on communist rebels waging one of Asia's longest insurgencies to hold their own guerrilla court hearings and then carry out executions.
The communists' armed wing, the 4,000-strong New People's Army, is known for assassinating civilians deemed to have committed so-called crimes against the people.
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However its courts and summary executions are illegal.
In a late-night speech on Thursday to a slum community in Manila attended by only a few journalists, Duterte called on ordinary Filipinos to kill drug addicts, which is also illegal.
Duterte also vowed to kill drug traffickers, using foul language he promised in recent weeks to "tone down" as president.
"These sons of whores are destroying our children. I warn you, don't go into that, even if you're a policeman, because I will really kill you," said Duterte, 71.
A former mayor of the southern city of Davao, Duterte had alternately denied and confirmed running death squads there that summarily executed suspected criminals.
Human rights groups say the squads -- made up of local police, former communist rebels and hired assassins -- have killed more than 1,000 people.
"But now he is in power, he needs to lend substance to those words and break with his earlier rhetoric," Rafendi Djamin, the rights group's director for South East Asia and the Pacific said in a statement.
Before dawn on Friday a bullet-riddled body was found in a Manila slum with a sign on it that read: "I am a Chinese drug lord," local police said.
The murder bore all the hallmarks of an extrajudicial killing, which would make it the first of Duterte's presidency.