A retired Philippine police officer has linked President Rodrigo Duterte - during his time as a mayor of Davao - and his men to nearly 200 killings that the officer and other members of a "death squad" allegedly carried out.

Arturo Lascanas made the allegations at the start of a nationally televised Senate inquiry on Monday after he admitted to lying in October during another Senate inquiry into alleged extrajudicial killings linked to Duterte.

Lascanas said that he had personally killed 300 people, about 200 as a member of a "Davao death squad", with his last in 2015. He also detailed two cases where he had murdered critics of Duterte, under the instruction of the then-mayor's bodyguard.

Lascanas, who broke down in tears before the media when he revealed his story two weeks ago, is the second person to testify before politicians to Duterte's alleged links to a clandestine hit squad.

"I feared for the life of my loved ones," Lascanas said when asked why he had earlier denied the death squad existed.

He said he changed testimony because he was tormented by what he had done and wanted the truth to "set me free".

It was "because of my desire to tell all the truth, not only because of my spiritual renewal, but the fear of God, I wanted to clear my conscience", he said.

Duterte's allies dismissed the claims as a plot by his opponents to discredit the president and his war on drugs, a campaign that critics say has disturbing similarities to a pattern of mysterious killings in Davao.

Duterte's spokesman, Ernesto Abella, described Lascanas as a "polluted source and perjured witness".

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Abella called Lascanas' testimony "fabricated and unacceptable".

Duterte has repeatedly denied involvement in summary executions, either as president or during his 22 years as Davao mayor.

His police chief Ronald dela Rosa, a former Davao police chief, has dismissed the death squad claims as a myth created by the media.

Human rights groups have documented about 1,400 suspicious killings in Davao while Duterte was mayor and critics say the war on drugs he unleashed as president has the same hallmarks. Numerous investigations have found no proof linking Duterte to those deaths.

More than 6,000 people have been killed in Duterte's war on drugs since he took over as president on June 30 last year.