Global human rights groups on Friday called for immediate release of a Philippines senator who is a known critic of President Rodrigo Duterte.

Leila De Lima has been in pretrial detention for two years now on accusations related to drug trafficking.

"The Philippine government should drop the politically-motivated charges against Senator Leila de Lima, a prominent critic of President Rodrigo Duterte's abusive 'war on drugs'," Human Rights Watch, Amnesty International and FORUM-ASIA said in a joint statement.

The three right groups claimed that the case of the senator was "emblematic of the deteriorating situation for all human rights defenders in the Philippines" under Duterte, whom they accused of stifling dissent.

"The government's fabricated charges against her only reflect how compromised its institutions have become under the present administration," said the joint statement.

After many delays and appeals, the senator's trial eventually began on Friday with the first oral hearing.

The accusation is solely based on statements of drug traffickers who are detained.

The defense team claims that the traffickers have been lured into giving false testimonies in exchange for lighter sentences, with an aim of framing the senator.

De Lima has been an eminent lawyer and activist who served as minister of justice between 2010 and 2015.

In 2016, when Duterte had completed a little over a month in office, De Lima moved a motion in the Senate to probe extrajudicial killings in the controversial war on drugs launched by the president.

In May 2009, as Chair of the Philippines' Commission on Human Rights, De Lima had opened an investigation into extra-judicial killings allegedly perpetrated by the so called "death squad" in the city of Davao when Duterte was mayor of the city.

Since coming to power, Duterte and his allies have responded with a "smear campaign" against De Lima, which included a video of a sexual nature allegedly with her chauffeur, and accusations of ties to drug traffickers based on false statements.

The accusations related to drug trafficking resulted in her being under arrest since Feb. 24, 2017.

"This is a blatant attempt to silence her courageous voice as she continues to speak out against widespread human rights violations, including thousands of extrajudicial executions of mostly poor and marginalized people," the statement from the rights group said.

According to the three organizations, the authorities have repeatedly violated the human rights of the senator, who has been practically isolated at the Camp Crame police headquarters, where she is rarely allowed to meet her visitors.

Delegations from foreign governments have been prevented from seeing her and she has been denied using electronic devices.