The highest-profile critic of Philippine President Rodrigo Duterte's brutal drug war was arrested Friday on charges she said were meant to silence her, but she vowed to keep fighting the "sociopathic serial killer". Speaking to journalists minutes before armed police in flak jackets detained her, Senator Leila de Lima insisted she was innocent of the drug trafficking charges that could see her jailed for life. "It is my honour to be imprisoned for the things I am fighting for. Please pray for me," De Lima, 57, said outside her Senate office where she had sought temporary refuge overnight after an arrest warrant was issued on Thursday. "They will not be able to silence me and stop me from fighting for the truth and justice and against the daily killings and repression by the Duterte regime." De Lima also recorded a polemical video just before her arrest in which she called for Filipinos to show courage and oppose Duterte's drug war, which has seen more than 6,500 people killed since he took office eight months ago. "There is no doubt that our president is a murderer and a sociopathic serial killer," she said in the 10-minute video that was posted on her Facebook page. De Lima, a former human rights commissioner, said her arrest was an act of revenge for her decade-long efforts to expose Duterte as the leader of death squads during his time as mayor of southern Davao city. Duterte first raised allegations in August that De Lima had been running a drug trafficking ring with criminals inside the nation's biggest prison when she was the justice secretary in the previous government. "I will have to destroy her in public," Duterte said then as he began a campaign to tarnish her reputation, including by making unsubstantiated allegations about her sex life. - 'People are afraid' - "De Lima is not only screwing her driver, she is also screwing the nation." The senator was last week charged with three counts of drug trafficking, and the arrest warrant was issued on Thursday. De Lima was taken Friday to a special detention centre for high-profile prisoners at the national police headquarters. The detainees' rooms are spartan, but are comfortable compared with the nation's notoriously crowded jails. De Lima and her supporters insisted that Duterte orchestrated the charges not just to crush her opposition, but also to intimidate anyone else who may want to speak out against the president or his drug war. "People are afraid," Father Robert Reyes, an activist priest who spent the night at the Senate with De Lima and other supporters, told AFP after her arrest. "If the government can arrest a powerful person like her, what chance does the little man have? That is the implied message of her arrest." Vice President Leni Robredo, a member of De Lima's opposition Liberal Party and elected separately from Duterte, described the arrest as "political harassment". Amnesty International said Thursday that it would regard De Lima as a prisoner of conscience. "The arrest of De Lima is a blatant attempt by the Philippine government to silence criticism of President Duterte and divert attention away from serious human rights violations in the 'war on drugs'," it said. United Nations human rights chiefs also expressed concern. "We are following the situation very closely," said spokeswoman for the UN High Commissioner for Human Rights, Ravina Shamdasani. Duterte's aides insisted they had a strong case against De Lima and said her arrest showed even the most powerful people would be brought to justice if they broke the law. "The war on illegal drugs targets all who are involved and the arrest of an incumbent senator demonstrates the president's strong resolve to fight pushers, peddlers and their protectors," presidential spokesman Ernesto Abella said. Duterte, 71, won the presidential election last year after promising during the campaign to eradicate drugs in society by killing tens of thousands of people. He launched the crackdown immediately after taking office in June and police have reported killing 2,555 drug suspects since then, with about 4,000 other people murdered in unexplained circumstances. Amnesty has warned that police actions in the drug war may amount to crimes against humanity. Duterte has repeatedly urged police to kill drug addicts as well as traffickers. But his aides insist he has never broken any laws.