Leila de Lima, who was arrested in February on drug charges after leading a Senate probe into Duterte's drug war, said that the allegations by the two officers had revealed "the ugly and disturbing truth of what has become" of the Philippines police.
Seventy-eight percent of 1,200 people surveyed by Social Weather Stations (SWS) said they were satisfied by the government's crackdown on illegal drugs, down from 85 percent in a similar poll in December past year.
The number of dissatisfied respondents rose from 8 per cent to 12 per cent.
Nearly 9,000 people, many small-time users and dealers, have been killed in the Philippines since Duterte took office on June 30.
The police have acknowledged that more than 2,600 suspects have been shot dead over the past nine months, but maintain most of those shootings happened after a suspect opened fire on undercover officers trying to catch them dealing drugs.
Philippine Senator Sherwin Gatchalian called for an investigation Wednesday into allegations made by senior officials in the Philippine National Police that officers have been offered cash rewards to execute individuals who are suspected of illegal drug activity. Human rights groups believe numerous remaining two thirds were killed by paid assassins cooperating with the police or by police themselves, disguised as vigilantes.
The SWS poll was conducted March 25-28, and also found that 92 percent of Filipinos said it was important to capture drug suspects alive rather than kill them.
"We are urging the Philippines to follow up on its commitment to investigate extrajudicial killings whether they are committed by law enforcement, or of a vigilante nature", he said.
The SWS survey on the anti-drugs campaign included questions on "extrajudicial killings", a term the government and police strongly object to, insisting no such killings have taken place.
Philippine President Rodrigo Duterte gestures as he delivers a speech during an awarding ceremony for outstanding Filipinos and organizations overseas, at the Malacanang Palace in Manila, Philippines December 19, 2016.
According to the report, 78 percent of the respondents supported the initiative, which represented a seven percent reduction compared to the opinions gathered in December 2016. He said he was angry about the impact of the killings on police discipline and wanted "to put Duterte on the defensive".
"This is a black eye for the Philippine National Police", said Ramon Casiple, head of the Institute for Electoral and Police Reforms. "They should do more and convince the public about reforms not by words but by actions".
The president's office, which said there was "no such report", added that the police were "not in the business of hiring assassins".