The Philippines derided the European parliament on Friday for interfering in its affairs after it issued a resolution calling for the release of a top critic of the president's war on drugs, which it said should target narcotics networks instead of users.

EU lawmakers on Thursday adopted a resolution condemning the "many extrajudicial killings" taking place in the Philippines and showing concern for the safety of Senator Leila de Lima, a fierce critic of President Rodrigo Duterte, who is being held on charges of involvement in the drugs trade.

"They should mind their own business," said Salvador Panelo, chief presidential legal counsel.

"They cannot dictate on the Philippine government on what to do with its constituent facing criminal charges," he said. "Nor can they can interfere with the judicial processes of our country."

More than 8,000 people have been killed since Duterte took power on June 30 last year and delivered on his election promise to launch a merciless campaign against crime and drugs.

Police take responsibility for over 2,500 of those deaths during their anti-drugs operations, but reject allegations by local and international human rights groups that police are involved in thousands of mysterious murders of drug users.

The EU parliament's resolution said it supported fighting drugs, but from the source, not the consumer.

It called on Manila to "prioritize" the fight against trafficking networks and drug barons over tracking down small-scale consumers.

Duterte's spokesman, Ernesto Abella, singled out the EU for criticism, rather than the parliament, and said it had been "spooked" into making "unwarranted threats" as a result of flawed information.

Duterte's ally and Senate President Aquilino Pimentel said the EU parliament was trying to "micro manage" the Philippines' internal affairs.

De Lima, a former justice minister, was last month arrested in her Senate office after being accused of accepting bribes from convicts engaged in the drug trade.

She says the charges are politically motivated and intended to stifle criticism of Duterte, whom she has called a "sociopathic serial killer".

The foreign ministry also rejected the resolution and said European lawmakers had no right to comment on the judicial system of a sovereign state.

"The Philippine government asks the international community to refrain from influencing the outcome of a case that is rightly under the jurisdiction of Philippine local courts," it said in a statement.

It said the government was "taking pains to investigate the veracity of these allegations of state-sponsored extrajudicial killings".