Convicted felons in the Philippines have testified before a Congress hearing that they had bribed a former justice minister and fierce critic of President Rodrigo Duterte.

The group of imprisoned gang leaders claim they were asked to raise cash through drug sales to fund Senate campaign of Leila de Lima, justice secretary in Benigno Aquino’s government and currently a senator's then senatorial campaign.

The testimonies dealt another blow to de Lima's efforts to investigate the country's extrajudicial killings.

She is now facing a hearing in Manila in response to the corruption accusations.

De Lima has called the Congress hearing on the drugs trade a sham [AP]

The hearing was held a day after de Lima, who is widely recognised as Duterte's main political opponent, was removed from a senate commission looking into extrajudicial killings during Duterte’s war on drugs.

The controversial campaign has killed at least 3,000 people in the past three months.

The senate inquiry, led by de Lima, included an account by former death squad member Edgar Matobato, who testified that Duterte was responsible for killings during his time as mayor of Davao City.

Duterte called Matobato's testimony a lie and de Lima was toppled from chairing the inquiry the following week.

'Long and lonely battle'

Herbert Colanco, one of the witnesses testifying against de Lima, says he received special treatment after paying her money earned through drug deals.

"For the 2016 elections, they [people working for de Lima] asked me to collect 30 to 50kg of crystal meth from Chinese druglords inside the prison," Colando said.

Antionio Trillanes, commision member, says the Senate should continue its hearings into Duterte's drug killings, although he admits this will be difficult.

"You're up against a president who has authoritarian tendencies and who is a self-confessed mass murderer," Trillanes said.

De Lima called the hearings on her alleged involvement in drugs trade a sham, and refused to attend. She accused Duterte of trying to silence her.

"I'm about to be crucified by the house of representatives," de Lima said.

"I knew this was going to be a long and lonely battle. But I will continue."

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Source: Agencies