A Philippine court on Friday acquitted of a drugs charge one of the fiercest critics of ex-president Rodrigo Duterte's "war on drugs", a move welcomed by activists who called her incarceration a vindictive effort to harass and silence her.

The court cleared former senator Leila de Lima, 63, of the second of three criminal charges against her, which stemmed from allegations made publicly by Duterte that as a justice minister in an earlier administration, she took payments from drug gangs in prisons.

De Lima has spent the past six years in detention, five of which as senator, and has one more case pending.

"I had no doubt from the very beginning that I will be acquitted in all the cases the Duterte regime has fabricated against me based on the merits and strength of my innocence," she said in a statement.

The charges came in 2017 a few months after she launched a senate investigation into Duterte's fierce crackdown on illicit drugs, during which thousands of users and dealers were killed, many by police or in mysterious circumstances.

The hugely popular Duterte responded by humiliating De Lima in public speeches with lurid revelations about her private life, prompting threats and online hate campaigns against her. He accused her of colluding with drug gangs in jails.

"I'm still asking for even more prayers for another case," De Lima said as she emerged from the courtroom and headed to a waiting police vehicle, as supporters chanted "Free De Lima now".

"Glorious day, glorious day, beginning of my vindication," she added.

Duterte has accepted the court judgement, Salvador Panelo, his legal counsel during his administration, said in a statement on Saturday.

"I have never interfered with the judicial process. I always say let the law take its course," Panelo quoted Duterte as saying.

Justice Secretary Jesus Crispin Remulla said the acquittal showed the independence of the judiciary.

"The rule of law has prevailed," Remulla told reporters. "Democracy is working."

Human Rights Watch Deputy Asia Director Phil Robertson said De Lima was a victim of a "vindictive campaign to destroy her" and called for the remaining charge to be dropped.

"Freeing her now is critical so she can return to her family, leaving the injustice of years behind bars in pre-trial detention caused by Duterte's vengeful cruelty," he said.

Amnesty International said the government should hold accountable those responsible for what it said was arbitrary detention and denial of her rights to presumption of innocence.

"The charges against Leila de Lima are bogus and the result of the peaceful exercise of her right to freedom of expression. She should not have spent a single day in jail," it said.

- Reuters