By Manuel Mogato

MANILA (Reuters) - Convicted Philippine felons on Tuesday told a Congressional hearing they had bribed a former justice minister and fierce critic of President Rodrigo Duterte, dealing another blow to her efforts to investigate the country's deadly war on drugs.

Senator Leila De Lima has become an increasingly isolated voice since seeking to hold Duterte to account for unleashing a crackdown on narcotics in which 3,800 people have been killed since the president took office 11 weeks ago.

Bank robber Herbert Colangco told Congress he had paid 3 million pesos ($62,637) a month since October 2013 to De Lima, then justice secretary, to let him hold concerts and sell beer to inmates.

He also said he ran a brothel and had been pressured to sell drugs to raise money for her election campaign, but had refused.

De Lima vehemently denies all the allegations against her, saying the witnesses were prisoners and former prison officials with an axe to grind or people who were being pressured by her political opponents.

"What the people have to realize is that this is not just an offense against me, but against everyone," she said in a statement.

"They are making me into an example of what will happen to those who dare criticize and call out the abuses of those in power. Who would dare stand up for others now?"

The congressional testimony came a day after senators voted to remove De Lima as head of a house committee probing the drugs war, a campaign condemned by the United Nations and human rights groups but cheered by Filipinos tired of crime and drug addiction.

"I have de Lima's blessings," Colangco said, when Congress members asked how he had smuggled contraband into the prison.

He said the bribes were paid through a security guard and presented what he said were receipts for the money remitted.

Another convict testified to seeing De Lima in the cell of an inmate who monopolized drugs deals in one of the country's largest prisons. Two former prison officials told the panel they had delivered 5 million pesos in bribes to De Lima's home.

Opposition to De Lima has intensified since she arranged for a self-confessed hitman to testify to a senate hearing last week that he saw Duterte shoot a man dead with an Uzi assault weapon and order other killings while a mayor of Davao City in the 1990s. Duterte's office denies the allegations.

De Lima did not attend Tuesday's congress hearing and gave a speech at the senate defending the investigation.

"No committee chairmanship is worth it, if it sacrifices my principles," she said.

Speaking at an army base in Davao, Duterte said the events in Congress showed there were plenty of people who could back up his allegations that De Lima was taking cash from drug gangs.

"I was correct all along," he said.

(Reporting by Manuel Mogato; Editing by Martin Petty and Clarence Fernandez)