Philippine drug war critic Leila de Lima was a highly paid lawyer who switched focus mid-career to human rights, a decision that would draw her into the deadly world of President Rodrigo Duterte.

De Lima was arrested Friday on drug trafficking charges she insisted were manufactured to silence her opposition to Duterte's anti-crime crackdown, which has claimed more than 6,500 lives in eight months.

De Lima, 57, began her career as one of the brightest legal prospects in the Philippines, having scored the eighth highest mark in the country's annual lawyer license exams in 1985.

She worked as a law clerk for the House of Representatives and as a legal aide of a Supreme Court associate justice, while building a successful law practice that specialized in helping prominent politicians in election court challenges.

De Lima helped now-Senate President Aquilino Pimentel 3rd to unseat a senate election rival, and future senator Alan Peter Cayetano beat a suit alleging he was not a Filipino citizen and therefore not qualified to run for office.

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• THROWBACK: De Lima won cases for Koko, Alan Peter as their lawyer

Ironically, Pimentel and Cayetano are now among the most powerful allies of Duterte, a longtime mayor of the southern city of Davao who won presidential elections last year after promising to kill tens of thousands of criminals.

De Lima's life turned in 2008 when she accepted her first government post as chairwoman of the government's independent Commission on Human Rights.

During two years in the job she worked with a small team of lowly paid lawyers, investigating extrajudicial killings, abductions and human rights violations by the Philippines' notoriously corrupt security forces.

Another one of her main targets was Duterte, who had for years faced allegations of running death squads in Davao that killed hundreds of drug addicts and petty criminals as part of a ruthless anti-crime campaign.

De Lima opened a probe into the so-called Davao Death Squads.

• Davao Death Squad killings still unresolved

She left the commission with the probe unfinished in 2010 to become justice secretary in the administration of Benigno Aquino, who swept to power on a pledge to fight corruption.

As justice secretary she prosecuted cases linked to the misuse of huge amounts of legislators' so-called pork-barrel funds, and raided the country's biggest prison to dismantle the perks accorded to inmates convicted of drug offences.

She left the cabinet in late 2015 to run for the Senate, winning a seat last year at the same time Duterte was elected president.

As Duterte launched a Davao-style anti-crime crackdown across the Philippines that would go on to claim thousands of lives, De Lima was one of the few politicians willing to criticize him.

"I will have to destroy her in public," Duterte said in August last year, and then proceeded to build a case alleging she was one of the nation's biggest drug traffickers.

• Duterte vows to 'destroy' female gov't official

"Throughout my entire life, I never imagined myself becoming a victim of a human rights violation myself," De Lima, a divorced mother-of-two, said in October as she insisted the charges against her were absurd.

Just before her arrest at the Senate on Friday morning, De Lima again insisted on her innocence while remaining characteristically defiant.

"They are mistaken if they are hoping that they will silence me and stop my fight for truth, justice and against the daily killings and repression by the Duterte regime," she said.

• De Lima: It is my honor to be imprisoned for what I am fighting for

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© Agence France-Presse