MANILA (Reuters) - Philippine senators on Monday ousted a staunch critic of President Rodrigo Duterte from the leadership of a panel investigating an explosion of drug-related killings, squashing a rare domestic challenge to the bloody crackdown.

Philippine President Rodrigo Duterte arrives at the military's Camp Tecson to talk to soldiers in San Miguel, Bulacan in northern Philippines September 15, 2016. REUTERS/Erik De Castro

Duterte’s war on drugs, the key plank of a poll campaign likened to that of U.S. presidential candidate Donald Trump, has claimed more than 3,800 lives from July 1 to Sept. 18.

The high toll and mysterious circumstances of some of the killings have alarmed rights groups, the United States and United Nations, whose concerns have drawn angry rebukes from Duterte.

The justice and human rights panel, chaired by former Justice Secretary Leila de Lima, was due to hear testimony on Thursday from a self-confessed assassin who last week alleged Duterte had personally ordered hits and shot a man dead during his more than two decades as mayor of southern Davao City.

Legislators voted 16-4 on a motion brought by senator and boxing icon Manny Pacquiao, with two abstentions, to unseat De Lima after allies of Duterte - including his former running mate - said her probe had dented the country’s international image.

Her removal demonstrates how much clout Duterte has won in his short time as president, and how dissenting voices against his anti-narcotics campaign are increasingly marginalized.

“What is destroying the reputation of this country are the killings. It’s not me,” De Lima told reporters as she walked out of the senate chamber.

The scope of senate inquiry is now uncertain and a separate probe will begin in the lower house on Tuesday, where witnesses are expected to give testimony linking De Lima to involvement in the drug trade, an allegation Duterte has himself made.

De Lima, who described the congressional hearing as a “sham” and predicted the testimony would be “lies and fabrications”, will remain a committee member, but was replaced as its head by Senator Richard Gordon, an ally of the president.

On Sunday, Duterte said he would be seeking a six-month extension for a campaign that had exposed a drugs problem far more entrenched than he initially thought.