European legislators on Thursday called on the Philippines to immediately free a senator who has waged a decade-long bid to expose hardline President Rodrigo Duterte as the leader of death squads.

Human rights activist Leila de Lima was arrested last month for drug trafficking, in what her allies say were trumped-up charges.

She has said the charges are to silence her probes into thousands of killings allegedly orchestrated by Duterte when he was a city mayor.

The often volatile Duterte won presidential elections last year after promising to eradicate drugs in society by killing tens of thousands of people and reinstating the death penalty.

MEPs in a resolution called "for the immediate release of Senator Leila M. De Lima and for her to be provided with adequate security whilst in detention."

They also strongly condemned "the high number of extrajudicial killings by the armed forces and vigilante groups related to the anti-drug campaign".

The MEPs said they felt "grave concern over credible reports to the effect that the Philippine police force is falsifying evidence to justify extrajudicial killings," the resolution added.

Since Duterte took office in the middle of last year, about 7,000 people have been killed in a crackdown on crime, prompting accusations by rights groups that the president could be overseeing a crime against humanity.

On Thursday Duterte's ruling party stripped a dozen lawmakers of congressional leadership posts for voting against the death penalty, turning up the pressure on his opponents.