On Monday, Duterte said that when mayor of the southern city of Davao, he would patrol the streets "looking for an encounter so I could kill
Since taking power in June, Duterte has waged a brutal "war on drugs" that has been linked to more than 5,900 deaths in less than six months
The President's comments are tantamount to an admission of "mass murder," Senator Leila De Lima told CNN Thursday.
"These are mass murders," she said. "High crime is a ground for impeachment under (the) constitution."
"We Filipinos are god fearing because we're a Catholic country and therefore we all know the killing is bad, killing is insane, killing is against the law of both man and the law of god," De Lima said.
Amnesty International regional director Rafendi Djamin said that Duterte's rhetoric took "the meaning of 'state-sanctioned' violence to a whole new level."
"By boasting about the blood on his own hands, President Duterte will further embolden police and vigilantes to blatantly violate laws and carry out more extrajudicial executions without fear of being held to account," Djamin said in a statement
Justice Secretary Vitaliano Aguirre defended Duterte Wednesday. "Just because you kill suspects does not mean you have violated the law," he said.
"It could be done with a justifiable cause and justified circumstances as a public officer in order to arrest, but the suspect fought. He must have been forced to kill."
In September, De Lima was ousted as chair of a Senate hearing into the drug crackdown and found herself under investigation in the House of Representatives for alleged corruption offenses.
She received death threats and was forced to leave her home
after a witness in the hearing against her read out her phone number and home address on national television.
"I don't feel safe," De Lima said Thursday. "I've been warned by some people that there's a serious security threat against me because of my vocal criticism of what the President is doing."
In the Senate hearing, a witness of De Lima's said he was a former member of the Davao Death Squad, a vigilante group linked to hundreds of killings during Duterte's tenure as mayor, and accused the now President of personally executing a government official
Duterte was elected in a landslide in May
and has maintained very high approval ratings
since, though some analysts have questioned the validity of this, pointing to similarly high ratings for his predecessors
While the President and his drug war are popular with many Filipinos, criticism of him has been growing in recent months.
There were mass protests
in November against the reburial of former Philippines dictator Ferdinand Marcos in a hero's cemetery, a move that was pushed for by Duterte.
Earlier this month, Vice President Leni Robredo resigned from a cabinet
post after what she described as repeated attempts to prevent her from doing her job.
Robredo was elected separately from Duterte and belongs to the opposition Liberal Party.
She told CNN Philippines
that Duterte had "the mandate of the Filipino people," but called for "passionate dissent" against his policies.
De Lima said that Duterte had committed a "betrayal of public trust."
"We do have an unfit President," she said. "He's the one promoting, encouraging and tolerating the killing."