"I don't feel safe. The truth is I am not safe," Senator Leila de Lima told reporters on Thursday.
De Lima was dramatically removed as head of the Senate Justice and Human Rights committee -- which is investigating extrajudicial killings related to Duterte's anti-drug war -- earlier this week.
De Lima said she was not confident whether Philippines security agencies would protect her.
"Can I rely on the government and (Philippines National Police) for safety or (National Bureau of Investigations) for my security? Can I rely on the (Armed Forces of the Philippines) to give me security? What is my choice?"
"It is unfortunate that opposition to this administration would amount to a threat to one's life," Edwin Lacierda, a former spokesman for President Benigno Aquino, told CNN.
"This is anathema to a genuine democracy where constructive criticism and a vibrant opposition are fundamental ingredients to living, breathing democracy."
During the hearing into corruption allegations against De Lima, which was broadcast live on multiple television networks, a witness read out De Lima's personal phone number and home address.
Since the hearing -- which she has called a "blatant exercise in harassment and persecution" -- she says she has been bombarded by threats and abusive messages and has had to temporarily move out of her house.
She is accused of accepting millions of pesos in bribes from drug lords in the New Bilibid Prison in return for political favors. De Lima denies all the accusations.
On Thursday, De Lima's former security aide Jonel Sanchez, who was accused of being her "bagman" by a witness at the House hearing, was confined for his own safety by the Presidential Security Group, according to CNN Philippines
"We immediately placed him confined in the barracks so that we can give way for the investigation," PSG spokesman Ltc. Michael Aquino said.
Mark Beeson, professor of international politics at the University of Western Australia, said the investigation into De Lima was "another effort to close down criticism of (Duterte's) administration."
"It's not encouraging that any kind of independent voices are being marginalized," he added.
Senate hearing resumes
The Senate hearing into Duterte's drug war resumed Thursday afternoon local time, without De Lima as chair.
Star witness Edgar Matobato reappeared before the committee after his testimony last week implicated Duterte in a spate of vigilante murders carried out by the "Davao Death Squad" when Duterte was mayor of the southern Philippines city.
In opening proceedings Thursday, new chair Richard Gordon said "we do not have to look far into the past to know the horrors drug problems can bring."
"We do not want to end up like Colombia," he added.
Senator Alan Cayetano cross-examined Matobato for much of the proceedings, saying the self-professed hitman's testimony regarding Duterte was based on "assumptions," not actions he witnessed himself.