MANILA—Senator Antonio Trillanes' determined effort to undermine President Rodrigo Duterte started way before he assumed the highest position of the country.

Speaking to DZMM on Saturday, Trillanes said he knows Duterte "personally" and what he found out in recent years came as a shock to him.

"Kinilala ko ito. Nakilala ko ito personally at marami akong impormasyong nakuha sa kanya, kaya noon pa lang hinaharang ko na pagkapanalo nito, kasi alam kong magiging disaster ito sa ating bansa," Trillanes said.

The opposition senator, who ran for vice president last year but lost, recalled talking to Duterte in 2015 and the subject of killing people being allegedly brought up.

"Parang kinilabutan ako para sa bansa. Parang may problema itong mama na 'to. Iyong utak niya pang-hitman," Trillanes said.

"Kaya eventually noong nag-decide siyang tumakbo, talagang ginawa naming misyon na harangin ito dahil maraming mamamatay and true enough nandito tayo ngayon."

Duterte won the May 2016 national election on a platform that focused on dealing with illegal-drugs issues.

During the presidential campaign, Trillanes accused Duterte of not declaring at least P211 million in his 2014 Statement of Assets, Liabilities and Net.

Earlier this year, the senator presented documents claiming that bank accounts of Duterte and his three children, and some under the name of the president's partner, Cielito Avancena, contained about P2 billion from 2006-2015.

Trillanes has stood by allegations of self-confessed hitmen Edgar Matobato and Arturo Lascañas that Duterte ordered killings during his stint as Davao City mayor.

The president has ignored accusations linking him to a so-called Davao death squad and has dismissed Trillanes' allegations, the most recent of which are corruption claims against Duterte's son, Davao City vice-mayor Paolo Duterte, and son-in-law Manse Carpio.

• READ: Duterte defends son, son-in-law anew and calls Trillanes' corruption allegations 'trash'

On Saturday, Duterte reiterated that he will step down if corruption accusations against his children are proven.

"Basta may corruption kay Inday (Davao City Mayor Sara Duterte), Pulong (Paolo Duterte) at Sebastian, bababa ako sa pwesto," Duterte said during a speech he delivered at the birthday celebration of Davao City Rep. Karlo Nograles.

Meanwhile, Trillanes, who is set to end his senatorial term in 2019, said he plans to take a break and go back to school.

"I'm going to take up another masteral course," he said.