A Philippine senator and staunch critic of President Rodrigo Duterte's war on drugs was in police custody on Friday following her high-profile arrest for drugs offences that she described as a vendetta that would fail to silence her.

Leila de Lima, who last year led a Senate probe into alleged extrajudicial killings during Duterte's anti-drugs campaign, said the arrest was payback for taking on a president who had acted like a dictator.

On Tuesday she called Duterte a "sociopathic serial killer" who had a "criminal mind".

"The truth will come out at the right time," de Lima told reporters outside the Senate office where she spent the night, moments before law enforcers marshalled her into a waiting van.

De Lima, her former driver and bodyguard and a former prison official were ordered arrested after a judge found merit in criminal charges filed by the justice ministry last week.

    She faces two more drug-related charges in the same court and described the cases as "all lies".

Bail is not permitted under the charges and if found guilty, de Lima faces a maximum sentence of life imprisonment.

House speaker Pantaleon Alvarez, a close ally of Duterte, described her arrest as a victory of the war against drugs, adding "no one is above the law, not even a senator".

But de Lima's supporters quickly came to her defence, with Vice President Leni Robredo describing the arrest as "political harassment".

Senator Paulo Benigno Aquino, a cousin of former president Benigno Aquino, called it "a concern for anyone who will dissent on any of the policies of this administration".

The criminal complaint alleged de Lima received 5 million pesos ($99,850) from a former prison official when she was justice minister between 2010 and 2016.

The allegations she was in cahoots with drugs gangs surfaced when she led a Senate investigation, which probed alleged summary executions during Duterte's bloody drugs war and a pattern of similar killings over the 22 years in which he was mayor of Davao City.

That investigation found no proof of wrongdoing by Duterte, who disparaged de Lima almost daily in televised speeches in which he made lurid allegations about her private life and even suggested she hang herself.

She filed a complaint with Supreme Court to try to muzzle the president.

At the heart of de Lima's campaign has been the 7,700 deaths since Duterte took office eight months ago, more than 2,500 in police operations. The cause of many of the other deaths remain in dispute and human rights groups believe many of them were extrajudicial killings.

De Lima was removed as head of her Senate probe by Duterte's allies and days later came under investigation herself in a congressional inquiry in which witnesses, several of them convicts, identified her as a key player in the narcotics trade.

Phelim Kine of the New York-based Human Rights Watch said Duterte had "effectively expanded his 'drug war' from the urban poor to the legislative branch" by arresting de Lima.