President Rodrigo Duterte has taken aim at the harshest critic at home of his anti-crime campaign, denouncing Senator Leila de Lima as an "immoral woman" for allegedly having an affair with her driver.

On Wednesday, Mr Duterte, 71, accused Ms De Lima, 56, of "political posturing when you yourself have a very sordid personal and official life". He alleged that Ms De Lima's unnamed, married driver was known to be collecting - on her behalf - payoffs from drug kingpins serving time in jail and claimed to have evidence to back this up.

"Here is a woman who funded the house of her lover, and yet we do not see any complaint about it. The money came readily from drugs," he said before hundreds of policemen on the anniversary of the Philippine National Police.

Ms De Lima, a former justice minister, fired back yesterday, accusing Mr Duterte in a news conference of "abuse and misuse" of power. "He should have double-checked the source of the information. That's really foul. I don't want to dignify that," she said of his allegation.

She said Mr Duterte's attacks might have been provoked by an upcoming Senate probe, which she would lead, into extrajudicial killings purportedly sparked by his ongoing war on the narcotics trade.

Over 600 people tagged as drug pushers and addicts have been killed either by lawmen or unnamed vigilantes since he took office on June 30.

The feud between the two has been simmering for years. As head of the human rights commission from 2008 to 2010, she led investigations into over 1,000 summary executions allegedly committed by "death squads" said to be under the sway of Mr Duterte, who was then mayor of the southern city of Davao.